My phone is stuck on 3G

Replace the phone.  It's not compatible with AT&T service, and when At&t completes 3g shut down, it will lose service (no later than 2/2022).

These are phones that do work if you prefer non carrier:

Voice over LTE capable phones for AT&T

The phone has only 1 4g (not even listing LTE) band in common with AT&T.  AT&T 4g, is HSPA+, or it's 3g service, which is being shut down.    The only band in common with AT&T or even T-Mobile is band 5.  

So yes, it's a 3g phone in the USA. And when 3-g shutdown will have no service at all

https://www.gsmarena.com/zte_nubia_red_magic-9174.php

ive got the red magic 3 and my phone just got stuck in 3g.the only reason i went to att was because it was the only network that supported the phone.i guess its time to go back to straight talk

@fingers19   it's not compatible with AT&T HD voice service.

These are:

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/ecms/dam/att/consumer/help/pdf/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwiX-Jv8oYLtAhXQo1kKHYhHDGcQFjAAegQIARAB&usg=AOvVaw3sCP2kJRUevj0ARWKoc1nJ