A back-up modem is critical now that I'm that working at home.
1) Does the modem have to come from Telus, or can it come from a third party as long as it's the same device?
2) Is there a way to rent/purchase a spare modem from Telus online?
The ease of finding a backup modem depends on your internet connection, VDSL and bonded VDSL modems are difficult to find. If you are on fibre, it again depends on your setup, as some use an ONT between the Fibre and the router, and others run the fibre directly to the router, with no intermediary.
However, the modem is but one point of failure, so if it is truly critical to have continuous internet, you should be backing up with a secondary ISP.
It is unlikely Telus would provide a second modem, unless you were in the fortunate position of having an old one left behind on upgrade to a newer one.
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UPS is of limited value for residential since a power outage is very rarely limited to just your house.