Welcome to the community!
As of December 1, 2017 service providers are required to offer unlocked devices (see here). Since the device was first introduced prior to that date, it's possible the phone might have been locked when you purchased it. If that was the the case, the mobile providers would be required to provide the unlock code without charge. Locked devices purchased after the December 2017 deadline will usually have the unlock code written on the box at the time of purchase. You should be able to test whether your device is unlocked by inserting a non-Fido SIM.
What plan do you have with Fido? If it is a post-paid plan, you're likely already enrolled with Fido Roam. Any usage of your services will trigger the Fido Roam charge. If you do not wish to travel with Fido Roam, you will need to opt-out, at which point you would be roaming on a pay-per-use basis. You can view the rates here.
Hope this helps 😀
To make sure you're able to use your Canadian LG G6 (if you bought it in Canada), you would need to verify 2 things:
1) Contact customer service to make sure you have the unlock code of the device if an unlock code is required. You can contact us here to do so.
2) Verify the technical specifications of the Brazilian phone company to be certain that the frequency bands of the network over there are compatible with your device.
Hope this helps you in your quest!