SIM cards for monthly accounts can only be activated on Fido.ca via MyAccount or in a Fido store. For prepaid accounts it can only be done in a store.
To do it on Fido.ca, sign-in to your account. You'll first be redirected to your Account's Overview. From there, click on Show all lines (if you have multiple lines) > Select your phone number from the list > Click on View Usage & Manage > Scroll down to Quick Actions > Click the link Update SIM Card.
Keep in mind that for a postpaid account, if you do not have the old SIM Card number then activation on Fido.ca will not work. You must pass by a store to activate your SIM card.
Hope this helps! 😀
***Updated on June 24th 2021***