OK.
EDIT: It is highly likely due to our ISPs using Carrier-grade NAT which is typical with 4g providers. I’ve checked and mine does use CGN. You can see that by comparing your public IP with your WAN IP (on the info page of our router). If both don’t match, your ISP uses CGN and it’s probably why we won’t be able to use ARC as long as we don’t have a dedicated public IP (which doesn’t mean it has to be static but your router has to get an IP which is public). See Tailscale implementation with ARC to circumnavigate ISP CGNAT for a possible workaround.