Unlocked Samsung S23 Ultra

This isn't a phone problem.

Network speed is determined by location and tower connection. If AT&T isn't cutting it for you then go back to your previous provider.

Too bad you didn't ask here first could have saved you a whole lot of trouble.   Unless someone with The same plan in your location is having great service, it's not the phone or SIM.  

Troubleshooting is just busy work.   There's no point in doing a factory reset. The only thing that might get you better connectivity is to 'reset network settings'. The only thing you have to fix then is to sign back into all of your Wi-Fi 

The FCC regulates who can turn out how much power and in what location.

I had a coworker standing next to me at work with AT&T  same phone except it was a locked AT&T phone and he had great speed over 200mbs

@Jcerda72 

Unless you are both on the same plan, that doesn't count for much.

What plan are you on?  

At&t unlimited premium 

Well clearly doesn't know an s23 ultra is a 2023 phone. 

Are you still getting messages that a SIM card that you're using is not a Verizon SIM?  

When you reboot the phone is it booting with the Verizon red circle, the AT&T blue globe, or something else?

My phone is an unlocked phone I was using it on Verizon with no problem, it has no carrier splash screens. I purposely by my phones unlocked from Samsung.

Gotcha, good.

Unlocked NA version should work as well as an att model.  Including Wi-Fi calling and 5g.  

Have you already talked to advanced tech support?  (Not the person who had you factory reset)