Comment by Dagger2

Make sure they actually have GUA addresses, not just link-locals.

If you're on a Linux machine, check `ip -c addr show` for an address that's "scope global" and doesn't start with "f". Those are the ones you need. If you have one of those, check `getent ahosts google.com` to see if v6 is being sorted before v4 in DNS lookups, and then `wget google.com` to see if wget prints any errors connecting to the v6 address.

If you have GUA addresses and nothing is outright broken, devices and software that support v6 will prefer it over v6.