Travel to Canada

The device you are using does make a difference.  Bell/Telus use a lot of mid-band for connectivity.  That doesn't work too well on overly congested networks.  This is not a T-Mobile issue.  It is the roaming partner in combination with your device.  If you're unlucky enough to have an iPhone with an Intel modem.  That is probably the main issue but the sparse deployment of Canadian networks doesn't help with weaker cellular modems.

I don't have any complaints when I am up I'm Canada but I am using an s23 Ultra.  I usually travel between Niagara Falls, St. Catharines, Oakville, Mississauga, Innisfil, Toronto, Cobourg, Peterborough, and Belleville when in Ontario.  Sometimes London, Brantford, and St. Thomas as well.