Welcome to the community!
The Fido Roam charge incurs when you use your Fido Services abroad. That is, the charge will incur when you make/receive calls, send text messages or use data outside of Canada (see here).
Generally using Wifi for regular internet access will not incur Fido Roam charges. In order to prevent unintended roaming charges, it is usually recommended to enable Airplane or Flight mode for the duration of your trip. You could then manually enable Wifi when required.
That said, there seems to be some misunderstanding regarding Wifi-calling. I'm not sure how people have gotten the impression that Wifi-calling does not use the networks. However, that is not true. While the calls and messages do not transmit via cellular towers, they still use the networks via the internet gateway (see image here). The cellular towers and Wifi are merely alternate means of accessing the networks. Calls and messages would not be able to complete or get sent/received without the networks. As such, using Wifi-calling is technically using Fido services. However, they have allowed certain usage to be free from charges. All Wifi-calling messages and calls you make to a Canadian phone number while abroad won't incur long-distance or roaming charges (see link above).
If you do later choose to use your Fido services, you should note that the US has shut down their '3G' networks (see here and here). You would need to ensure your device and plan are compatible with voice-over-LTE (voLTE) in order to make/receive calls on the US networks.
Hope this helps 😀