@hisaful
Critical part to US roaming depends on T-Mobile/AT&T/Verizon coverage and whether you can connect successfully onto either network.
If the phone is not connecting to T-Mobile/AT&T/Verizon, change the Network Connection from Automatic to Manual and select and test each of the three and see which one you can connect and work best
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)
