Hi @Andy_Jones,
Prefix delegation might be the culprit in this case, since you have an additional Unifi router in the setup. What happens if you set IPv6 Interface Type to Static in the above screenshot?
Thank you for volunteering your time. If you’re eager to get access to ARC, since you’ve been stranded so far, we recommend the TailScale workaround in the meantime: Success with Tailscale VPN