That's super weird for one of them to all of a sudden have trouble guslopez15.
Can you double check the APN settings on it to make sure everything's correct? If you do have to make any changes, I suggest giving the phone a restart then testing again. Another step you should try is testing the SIM from that phone in the other that does work to see if the issue follows. Please let us know the results.
Just wanted to check in and see if you'd been able to get things fixed!
I realize this is older, but you're probably being plagued by the September update. In addition to complete signal loss I've had this update disable my data after a reboot. You just need to go to settings -> data usage and turn mobile data back on. There's an entire discussion on this issue as well at This link https://support.t-mobile.com/message/587342#587342