CH -

You offer very little about yourself or your experience in this environment.
I assume the "GB" means Great Britain - which has a very different September.
Also, you don't say whether you plan to start in Alberta or Montana.
The former is closer to 6 weeks, the latter 4 weeks - big diff in the fall.
Plus, days are shortening rapidly by September.

I have lived in Wyoming and Montana for almost 30 years.
Have cycled in all seasons and all parts - from pavement to remote single-track.
Late Sept I have often encountered -10C - and almost always have heavy ice on my tent fly.
And fast-moving early storms can dump a foot of snow overnight - when a dusting was predicted.
That can close backcountry forest roads for the season -

Although hunters may use snowmobiles or four-wheelers to get back in.
Speaking of hunting - you need to have some highly visible orange.
It will be hunting season - and hunters can be of great help if you encounter difficulties.