Hi- I brought my own unlocked AT&T HTC One to T-Mobile by buying a SIM and starting service via the T-Mobile website. After installing the SIM, I set up the APN for the HTC One, as noted here:
Connections & network: HTC One (LTE)
(Everything, as listed, with: APN type: <Not set>; APN protocol: & APN roaming protocol: These listings don't appear in my APN settings. NOTE: if I set APN type: Internet+MMS, I lost all connections)
After doing this, I could place/receive phone calls, send/receive SMS text messages, and had a 4G/4G LTE (wavering between) data connection to the Internet. BUT, I couldn't send MMS messages (it tries sending for a few minutes, then stopa with an error), and when an MMS message was sent to me, it came in with a label of who it was from and an icon saying "Download". But, it wouldn't download when it was "clicked". The error given for both sent and received MMS was "General Network Failure". I used the texting app included with the phone.
Next, I tried sending/receiving MMS using Google Hangout as the default app, but it still wouldn't work (again, SMS sending/receiving worked).
Lastly, I tried the following troubleshooting suggestions:
Picture Message (MMS) troubleshooting: Android
So, I'm not sure what to try next. I should note that sending/receiving was done with another AT&T HTC One on the AT&T 4G LTE network, all photos were about 185 Kb (i.e. less than the max limit imposed by T-Mobile or AT&T), and the AT&T phone did appear to send the MMS message. Also, I tried the general APN settings given in the forums for generic Android phones, but that didn't work either (they are very similar to the HTC One version).
Thanks for any help you're able to give.