S7 Edge Verizon to ATT not working properly

[Android Central Question]

I recently moved from Verizon to ATT (Firstnet account). I installed an ATT SIM card in my Verizon S7 and updated the Access Point Names. I am finding that MMS is spotty as well as receiving calls. I can call out just fine, internet is fine, SMS is fine. Even with a good connection I'm not receiving all calls. Any advice? (Please keep it simple, because I am) PS, I've been a fly on the wall for years on this site, the members are very helpful.