Don't know if it helps you but under Cellular+SIM under SIM settings I have 4G for highest connection speed, automatic for network selection and T_Mobile for cellular connection profile. Everything is blank under MMS APN.
I was worried I lost mine after having T-Mobile set it up in a store. I don't know what if anything they actually did because I was there to get a nano-SIM so that was the first time it had a SIM in it.. But later, I reset the phone after installing DP 8.1 and thought I may have to do it again. But everything continues to work fine.