Friend of mine picked up an HTC One the other day off CL for Sprint and decided to use it for T-Mobile.
The person was using it on Sprints network before and so under Network Mode, "GSM/UMTS Only" had to be selected because it was still receiving signal from Sprint through CDMA, but not Data. He put his sim in and clicked on the "Available GSM Networks" and after searching for a minute, T-Mobile and AT&T were listed as options. He picked T-Mobile and it automatically put in the correct APN settings for him.
What's weird though is that when the phone is freshly turned on, Data works and it comes from the epc.tmobile.com APN. However, when wifi is turned on or the network is disconnected due to lack of signal, data doesn't come back on when signal arrives again and the phone automatically changes the TMobile Data APN to cinet.spcs (the original APN for data through Sprint). What's going on here? Any insight would be great.
What he'd really like to do is just do a complete restore using a TMobile HTC One boot loader, but is unsure how to go about this.