A US CFI can instruct in a US registered aircraft worldwide.

You would need a Canadian CPL to fly (and get paid) in a Canadian registered aircraft in Canada.

You would need a Canadian CFI to instruct in Canada in a Canadian registered aircraft.

If you can get the Canadian registered aircraft to the US, then your US CFI will allow you to instruct in that aircraft while in the US only.

You could not take the Canadian registered aircraft to Mexico without a Canadian CPL, and you could not instruct in that aircraft without a Canadian CFI.

Joe