Odd Android Hotspot Issue
 

I'm hoping it's something easy to fix and not a problem embedded in the (NTT Docomo) ROM, but when I share my data connection as a hotspot, my data connection drops as soon as another device makes a connection with my phone.

Turning hotspot sharing off restores the data connection immediately. Sharing worked in the past, so I am unsure what might have happened to create this issue.

I've tried sharing with different SIMs and networks - same problem.

Not much to go on here (like Android version and phone). But have you checked things like maximum number of connections?

Model: Samsung S4 SC-04E
Version: 4.4.2
SE for Android: SEPF-SC-04E.4.4.2.0016

The only thing enforced is a password on the wifi hotspot, but it's the data connection that drops - the devices still remain connected to the phone via wifi.

Then I suspect DoCoMo doesn't like multiple connections :(

The thing is I'm connected to ATT - so it could be the ROM is set to disable sharing? It worked before, maybe they updated it :( That would be a problem.

Sounds like a problem with a carrier specific rom. Even when a phone is unlocked, some of the system files are still custom to the original carrier. I recall reading this same issue, but with an AT&T S4 operating on another carrier. Phone itself would get internet, but you couldn't wifi or BT tether it to anything else.

Google "tetherprovision" and your phone model. Should find a few threads on XDA with directions, but only try it if you are comfortable messing with system files. Could easily brick your phone if you're unsure of what you are doing.

The details here are scant, but try something like the Foxfi or Wifi Tether Router apps from the Google Play store. I can definitely tell you that the carrier-specific roms have various limitations and restrictions.

If you are willing to root and or replace your current factory rom, your options and capabilities will expand greatly.