Welcome to the community!
What phone do you have? Also, what error do you receive? Is it a SIM-lock issue? If you purchased the phone from Fido, you shouldn't have any SIM-lock issues using it on the Fido networks.
In addition, as of December 1st, 2017, all phones sold in Canada should have been unlocked (or at least unlock code provided) (see Wireless Code). If you purchased your phone from Fido 2 years ago, it's unlikely they had any locked devices in stock. That said, if you did purchase a locked device, you should have already been provided with the unlock code (often written on the box).
If you still require the SIM to be unlocked, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀