LG G5 (AT&T Unlocked) Reboot Loop after inserting sim

[Android Central Question]

We just bought this phone on boxing day, unlocked but original carrier was at&t. Seems it's on version 7.0. Works just fine using straight Wi-Fi. We went and got a plan with our local company Sasktel. Inserted the SIM and was fine. Noticed it didn't allow any data for apps or browser, just text messages. Added the required APN from my carrier and the phone decided after half hour of amazingly having 4g that it was going to shut off, and get stuck in a boot loop. When it did start it wouldn't allow any wi-fi greyed it out. then stuck in a boot loop again. Reset phone , took forever to re set it up with multiple reboots in the process. I havn't found a method to remedy this yet. Have it straight on WiFi again with no SIM card and it seems stable but my son needs it for bus routes where there is no WiFi....I wouldn't care so much if it wasn't all his christmas money he spent on this damn thing so I have to try and fix it. I don't know much on Android but very capable of learning.