If you have ATT fiber, it’s a pain in the butt. Their default router will only issue a single passthrough /64 on request. If you have multiple VLANs you have to setup some scripts to ask for more, and even then you only get 8 of them. The gateway reserves the other 8 from the /60 it gets for its own use.
The only way I got IPv6 working well with them was to bypass their gateway. Now all my VLANs have /64, which is the standard subnet size.
I think bypassing their gateway, that is - bringing your own router is the only way to do VLANs, because their gateway is very basic and doesn’t support VLANs at all.
You can do VLANs with their gateway but only IPv4, or you have to write custom scripts to ask for additional IPv6 delegations.
Interesting. Which model of their gateway do you have? I have BGW320 and it definitely doesn’t support vlan tagging.