I suppose you should never blindly trust the competence of an ISP-provided router, but I would expect it to automatically block all incoming IPv6 traffic unless

I suppose you should never blindly trust the competence of an ISP-provided router, but I would expect it to automatically block all incoming IPv6 traffic unless you explicitly add forwarding rules. You can always enable v6, then see if you can reach internal machines from a machine on the internet (like a VPS or over a cell connection)