@Atk-47 - Are you sure that you indeed got a Canada US Plan? Check 'My Account' > Subscriptions & Add-ons, it will tell you what plan you are on - if it has 'Canada - US Data'
Make sure that 'Data Roaming' is on, if it is off then you cannot roam hence why your phone may not work in the USA.
If it indeed has the plan, move on to the next step, if not, buy a roaming add-on
Check what phone provider your phone is connected to
Some people have reported that the phone auto connects to Verizon Wireless, which PM has no roaming support for.
Try switching to Manual and selecting T-Mobile or AT&T.
Now by what you have told me: here is my main recommendation:
If nothing, check if your phone is VoLTE Capable: https://www.publicmobile.ca/en/on/get-help/articles/volte
If your phone isn't on the WhiteList then hence it isn't supported for Voice roaming in the states.
Try manually connecting to T-Mobile 2G / EDGE / GSM and see if it works. T-Mobile is the only carrier that still has a backup system incase devices aren't constable with their / roaming partners Voice Over LTE network.
