If you're just trying to put a new (well new to you) S21+ on a the Koodo network with your existing Koodo SIM card, you should be able to, although if you're moving from a 4G phone to 5G, you may need a new SIM for 5G speeds.
Plus, if it's a Exynos version of the S21+, just know that it's a slower modem to start with (despite slightly theoretical higher specs) even before it's non-NAM carrier focused BAND support. Meaning it will still support the basics to work, but might not be accessing the best BAND/TOWER available.
To check if your phone works on Koodo;
https://www.koodomobile.com/en/help/phone-compatibility
It'll take you to a Telus page, then click on the "Check if your phone is supported" drop-down and enter your IMEI
