There is some foreign competition via roaming, e.g. both T-Mobile and AT&T have US mobile postpaid plans with unlimited Canadian roaming, https://www.att.com/international/canada-roaming/ & https://www.t-mobile.com/cell-phone-plans/use-your-phone-in-...
For devices with low data (text/email/web) usage, there are vendors of global prepaid eSIMs that last 6-24 months, which roam on Bell and Rogers 5G networks, priced around $5/GB, e.g. Eskimo (Singapore) and AIS (Thailand) eSIMs.