Hello CCenci,
Welcome to the community!
To clarify, is the number you wish to forward to a Canadian or American number? Calls can only be forwarded to Country code +1 (see here). If you're attempting to forward your number to another Country, that could be why the codes do not work.
That said, attempting to enter the codes while outside of Fido/Rogers networks would be considered roaming. Since you are already outside of Canada, you would need to contact customer service to have it set up for you manually. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above (see here).
If you have previously availed of the service, you might already aware that Call Forwarding isn't free. Older plans included a certain number of Call Forwarding minutes (ie 2500 Call Forwarding minutes). However, newer plans don't include any Call Forwarding minutes. That said, 2500 Call Forwarding minutes is included with the Value Pack Add-on (see here, under Voice-mail). Fido might offer those Call Forwarding minutes as a standalone add-on, though you would have to request it via customer service.
Hope this helps 😀
Cheers
