@Ashbro - When I get IPv6 I know my clients have received GUA but many a times whatismyip shows IPv6 not detected at all. Still I can make inward connection using GUA to that client. So there indeed is public IP6. These sites are unreliable. What you can try is see if you can match the ip6 settings between zte and archer. that's what i had to do with D-Link router.
@su_mumbai - try disabling SLAAC in ip6 settings in WAN connection and check again.
@su_mumbai - try disabling SLAAC in ip6 settings in WAN connection and check again.