you don't need to roaming addon if you have the Canada-US plan
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
you need to puchase US roaming add-on or your plan has to be one of those Canada-US plan to use the service in US
As a workaround, download VoIP app Textnow and you can use it to make and receive regular phone calls on Wifi (using another 10 digits number you choose during setup). You can use the app to call yourself and check voicemail.