Welcome to the community!
Generally, Fido/Rogers customers will roam on either AT&T or T-Mobile networks as they operate UMTS (WCDMA) & LTE networks and cover most of the US (T-Mobile also has a limited GSM network). **edit** However, to which network your phone connects depends in part on the band/frequency capabilities of your phone.**
Both Sprint and Verizon operate (or at least did) using CDMA technologies which are not compatible with GSM phones.
Hope this helps