Go to Settings-General and see if there is a profile there and, if there is delete it and restart the phone.
Assuming you are in an area with a strong LTE signal (for instance you know others on Xfinity Mobile can get LTE at your location), here are a few things to try and check after each one to see if the situation has improved:
1) turn Airplane Mode on and then off again.
2) go to Settings-General-Reset and reset network settings
3) Turn off wifi
4) Turn off the phone, remove sim, replace it and turn the phone back on again.
If none of these help, make sure there is no carrier settings update available for your phone.
If you still have problems, check the APN settings by dialing *3001#12345#* and then selecting PDP Context Info and then 0.
When I dialed the number the only options were Device Info, LTE, UMTS, AND GSM. I went through each option and there was nothing I could hit that said PDP Context Info.