How to activate a BYOD phone with two IMEI's (2 sim cards)?

My phone has 2 sim slots (not e-sim), I 2 active sim cards. My AT&T prepaid & TracFone. I can receive and make calls and texts. Both cards have been active and working for a while.No issues with service. But I must activate the device so AT&T recognizes it in order to not lose my service . I keep getting alerts to upgrade my phone because the last phone I used before this one was my iPhone 3gs. So the system still thinks I using a 3g device. My current device is a Blu G5. Does anyone know where I can get this done? Perhaps a link you'd share here if you too had the same type issue? Thanks!