Eh,
Are you living close to the US border? Your phone can latch onto an American tower without you going anywhere, and that triggers the roaming notification. That notification is a generic notice though, it doesn't mean you're actually being charged. Have you checked your usage in Self Serve? any actual Easy Roam charge would show up there.
If a charge does land on an issued bill, you can submit it through Ask us to review a charge in Self Serve, accidental roaming like this normally gets credited https://www.koodomobile.com/en/help/ask-us-to-review-a-charge-on-your-bill
Which phones are you using? Koodo not showing in the network list is odd, and the menus are different on iPhone vs Android. For now I would turn data roaming off in the phone settings on both lines. You wouldn't be able to select Koodo if your area didn't have Koodo signal