Why won't my LG V20 register ATT network but it will register T mobile?

[Android Central Question]

I just purchased a LG V20 on Ebay. It says it is unlocked ATT phone. I have Net10 which uses other carriers towers for network. My SIM is ATT compatable. It will not register network connection at all when in the V20. If I put a Tmobile SIM in the phone it will actually register it. I even had Net10 send me a Tmobile SIM and it worked fine other than I get like no service with Tmobile. So I ordered a new ATT SIM. Put it in the phone and nothing. What is the deal. Should I be trying to send this phone back because I got a bad phone? Also the IMEI check was clean. This is an unlcoked ATT phone, so why the heck does it work with Tmobile and not my ATT SIM.

 

[Rukbat]

In order to register on AT&T, the phone has had to try registration on 3G since about the end of 2016. Unlocked AT&T phones will do that (and it works on T-Mobile, because they fall back to 2G registration for areas not yet converted - the newer phones won't). An unlocked T-Mobile phone won't register on 3G (they still use 2G registration - as does the rest of the world). But if you're on Net10, and you get T-Mobile signal wherever you go, there's no problem - just use a Net10 TMobile SIM. (If you needed AT&T, because you didn't get a T-Mobile signal where you are, you wouldn't be asking, so I'm assuming that you are getting a T-Mobile SIM.) "An AT&T phone" isn't a guarantee that the phone will work on AT&T in all their markets (or probably any of their markets by now).