Hi, are you sure your T-Mobile prepaid Sims are still valid/active (from last summer) with a balance on it ? Funds in the account typically expire after 90 days unless you a new refill is added and then the whole balance rolled over - I don't believe you have one of the legacy/grandfathered old T-Mobile Pay-As-You-Go and/or Gold account, where for $10 - your account remained in good standing for 360 days.

The following steps - Short Code check - might or might not work - on your smartphone, enabled WiFi - when you say you have no coverage, you might not be getting roaming coverage/valid U.S. band on your device ?? - and on the iPhone's dial pad, press
#BAL# [Enter] to check for your available balance
#MIN# [Enter] to see if you have any minutes remaining, more importantly - expiration date/next refill date

It should return the results on your smartphone's screen in a few seconds, before depositing money into the account/mobile # - already recycled & given/reused by someone else here in the US.

This number 1-505-998-3793 is used to call T-Mobile USA's international support line for help, from overseas. See if you can dial & reach it on your smartphone by dialing (enable roaming calls first and/or WiFi calling on your iPhone) They can lookup the mobile # and advise you the status, and should be able to help you to find a way to refill it.

If it doesn't work & you cannot get the Short Codes to work at all, have the Sim card's identifying #'s available and then call the above T-Mo # for status & update/refilling help.

Callingmart dot com is another option - upon payment, they can refill directly to your mobile #'s at T-Mobile, providing that you are sure your Sim cards are still valid, active and good standing & not expired already, etc. Been using Callingmart for 10+ years to refill some of our prepaid lines for the household/extended families & in-laws, etc.

T-Mo's link - http://www.t-mobile.com/contact-us.html