Reboot your phone WORST case make sure the APN is set to fast.t-mobile.com (you can also try epc.t-mobile.com )

Exact text on the error message would help also - thanks

Thanks- I've tried all the usual troubleshooting items- rebooting, verifying APN, etc.

The error message I'm getting on my iPhone is "Could not activate cellular data network- PDP authentication failure"

Unfortunately, there doesn't yet seem to be a report in this thread about data service working in Macau

I had similar problem last year when in Phnom Penh. My Moto G worked fine elsewhere in SE Asia during that trip but for some reason, couldn't get data connection for 36 hrs. On the way to airport to fly out, it suddenly turned on....it just happens sometimes????

OK this helps; please make sure LTE is NOT ON

Is this a grey market iPhone (carrier file may be out of date) [e.g. did you buy it unlocked in another region]

I know this sucks; but try the following as well

Cycle data: Do a #ROF# in dialer; wait for acknowledge, then try #RON# ; reboot phone

If still not working try a Power + Button hold down to reset phone (this has been reported to restart modem/radio more so than a power on/off and re-read settings)

FINAL; check T-Mobile has your account correctly setup; you can call intl support and have them try removing your data plan then putting it back on if this is THEIR system failing auth.

I just called roaming support for T-Mobile and they did confirm a 3G roaming agreement WITH data for CTM, Three and Smartone; can you try one of the other networks also?

Yup; seen this as well; don't know if it's T-Mobile side or the carrier roaming where auth fails. Noticed this tends to happen more often between 10PM and 5AM Eastern time if it happens at all so don't know if provisioning system has maint or downtime/slow response causing some roaming networks to not authenticate data. But as you pointed out it just starts working at some point.

The problem may be that your phone does not automatically set the APN for the service that you are connecting to. Often with older phones, one must actually manually set it for each carrier. Voice does not use an access point so works as does wifi in every country I have visited. Unlocked phones with local SIM often require the same APN input.

Actually had it happen here in the U.S. when I hooked up my LG AT&T phone on the T-mobile US network after porting over the number and cancelling ATT. Had T-mobile manually set APN and it has worked splendedly since.

The scenario you describe sounds like your phone didn't auto-provision when you changed SIM cards. But that's different from roaming, in which you shouldn't have to change APN (or SMSC) at all. If you do, then something is misconfigured at the roaming partner.

Data roaming problems have been encountered in Thailand, Phillipines, Australia, New Zealand, Czech Republic with two different phones (admittedly older models) which required the roaming partner to input the APN manually. Also in Bulgaria, France and Romania where I did not bother and just used wifi for data.

The phones always work with voice roaming. So I disagree with your analysis.

I have consistently had problems with Hong Kong more than any other locale using T-Mobile. I have been here since Tuesday and my phone was never able to make voice calls while registered on 3. I kept trying to manually select different networks the last few days, along with a call to Cust Care to try to resolve.

The phone would auto connect to 3, and data worked perfectly albeit a bit slow. However the phone would not make any voice connections and SMS would not come through. It's an iPhone 6 on ios 8.1.

I tried forcing it to SmarTone, China Mobile, China Mobile (3G), CSL and PCCW. None would work. It would always go to No Service. I tried multiple power off and power on cycles, and nothing worked.

Until just a few moments ago... Day 3 without phone service. I selected SmarTone HK and while it initially said No Service for a few minutes, once I powered off the phone then turned it back on (Normally, not hard reboot), it worked immediately on coming back up. Followed by a flood of SMSes that had backed up.

I am very torn with T-Mobile. This time the lack of voice access posed significant challenges. AT&T always just worked. I never had to do anything no matter where in the world I landed.

TMobile, however, is extremely buggy when it comes to registering with the roaming network. This is the third time in HK I've had data with no voice, other times I've had voice with no data. I've also had similar No Service issues in other countries around the world. I'm really torn between the cost efficiencies of T-Mobile and the solid reliability with AT&T. Might have to switch back for a while until T-Mo sorts this out.

My statement was that the need to manually input an APN (or SMSC) when roaming indicates a misconfiguration in the roaming partner's network.

Do you disagree with that just because you have a seemingly 'large' number of examples of such misconfiguration?

The other part was that SIM swaps and roaming are different scenarios. Which part of that do you disagree with?

I have hit half of the countries listed by poster you comnented on (and another 15 more) and NEVER changed my APN. I'm using fast.t-mobile.com as my APN and my account had data reprovisioned once. Since then NEVER had a roaming failure (other than the ones in the Wiki; e.g. Smartone takes a few attempts to get voice in addition to data)

I had to change my APN only once, with assistance from customer service, when I brought my Nexus 4 over from Straight Talk to T-Mo when Simple Global began. After that my Nexus 4 worked fine in multiple countries in Asia and Europe. I would occasionally manually select a network to see what worked best, but I never had any problems. Are people changing their APN's w/o contacting T-mo customer service, if so then I would guess that the problems are of their own making.

Agree: you should ONLY be using on standard plans EITHER;

fast-t-mobile.com
or
epc.t-mobile.com

Anything else may fail roaming international

This worked for me in the British Virgin Islands last year. Once I did THIS, data worked fine.

I saw this posted on the T-Mobile site.

--
Important information regarding Vietnam

On January 1, 2015, customers roaming in Vietnam will have their data experience slowed to 2G speeds. On January 13, 2015, data roaming access in Vietnam will be blocked. At this time, voice calls and text message (SMS/MMS) access is not scheduled to change. This was an independent move by the Vietnamese government, and T-Mobile is working to get the rates back down.
--

Guess I'll have to get a local SIM for data coverage there next month. I think the Vinaphone one has a package for 25MB per day for less than $1/day. And I was about to get rid of my 2-sim Samsung Core, I guess I'll keep it for awhile.

.

Unlocked hotspot is option for using local sim and still keeping a better device to use while out of country