@Jason_77 do you see the phone able to connect to either network at all??
If you decide to ask for reverse of the purchase instead, please message support
@Jason_77 still in US?? want to give it another try??
Critical part to US roaming depends on T-Mobile/AT&T coverage and whether you can connect successfully onto either network.
If the phone is not connecting to T-Mobile/AT&T, change the Network Connection from Automatic to Manual and select and test both T-Mobile and AT&T and see which one you can connect.
If your issue is with data, make sure your Roaming Mobile data is enabled and you have disabled your mobile data limiter. Also change your network mode to 4G (and don't try 5G until you can connect and works on 4G)
If your issue is with voice, make sure VoLTE is enabled on the device setting. And if it does not work, try to change the Network to T-Mobile ONLY, and change Preferred Network Type or Network Mode to 2G and see if the calls go through.
Also, you can download VoIP app TextNow as a workaround to make calls and text if you have Wifi or if your roaming has data component and data is working
Use this link to message the Customer Support and request a credit to your account for add-on that failed to work.
In future, make sure you have a volte enabled phone to make sure you have best calling experience. Ensure you have Roaming Data enabled.