Iphone 5 signal is E

We have 4 Iphone 5's in this house.. Only ONE of them will NOT get 4g. My son's phone stays on E most of the time. They have swapped the sim, and that worked for about 2 days and then back to E it went. Just two days ago they completely swapped his phone out and for less than 24 hours he had good signal and then back to E. WHY do 3 of us in the same house have 4g and his phone has E? Rebooted, restored, you name it, I think we have tried it and his phone just won't keep the 4g.. HOWEVER, he was on a trip last week and his phone got LTE for a week, and then back home to E. So his phone does work, but here where everyone else is getting 4g, his phone is not. Is it the program, the phone, something as weird as where his phone number is coming through, I'm grasping cause it makes no sense that 3 out of 4 phones work perfectly. Any ideas other than going back and getting another phone??