Their new modem/router doesn't support opening ports in the ipv6 firewall, so if you want to open ports, they recommend disabling ipv6 entirely. For ipv4, they no longer support forwarding ports from only specific source addresses either, which is way less secure. You can only forward ports from all source addresses. You also have to use their crappy app to add port forward rules, it's no longer available in the web ui. You can completely disable the ipv6 firewall in the web ui, but that wouldn't be safe.
Old motorola modem/routers could do all of the above.
It says it can do bridge mode at least, but it seems silly to need 2 devices just to open ipv6 ports.
How are routers being made now in 2023 that don't have proper ipv6 support? It seems crazy to me.
IMO if you're serious about IPv6 you should probably have your own router running OpenWRT or the like. That's not to say consumer routers don't exist with good v6 support. AT&T provided routers have very good v6 support including full firewall rules for both v4 and v6 on top of the v4 port forwarding for NAT. We'll ignore their PD issues lol. Sounds like Xfinity might just be behind the times. I'd put OpenWRT on a router and use that instead of the ISP router anyway.