tp link archer c7

I don't know how successful you'll be with IPV6 and the TP Link.  Looks a bit limited on advanced options, all you got is the canned firmware you got, and Xfinity is both goofy and undocumented on how they implemented their IPV6.  You might not have much for error logs either.  If you have any that can see the DHCP/DHCPv6 transactions, you're probably getting 82/24 timeouts on the lease renew (no communication with the server upstream). 

On BSD/Pfsense/Opnsense, the important bits are how you handle the IPV6 lease, prefix size and Router Advertisements, and internal DHCP servers (IPV4/IPV6).  You could use a linux box as a router/firewall too.

For the lease and prefix:  

delegation size = /64 (in my market)

If there's an option, Do Not Wait for Router Advertisements (fixes timeouts with flaky upstream servers)

If there's an option, Do Not Allow PD/Address release (holds your current lease if you can't contact the DHCPv6 servers reliably)

For the Router Advertisements, it's a bit strange.  It's not managed, and it's not stateless (SLAAC) entirely, it's a combination on Xfinity:

If it's an option, set Router mode Assisted - RA Flages [managed, other stateful].  If there isn't an option like that, you may be out of luck. 

For the DHCPv6 server internally, set a range like ::1000 to ::2000.  For the LAN interface, set IPv6 Configuration type = track interface, and set the WAN as the interface you want to track.  

You probably don't have that level of detailed configuration on the TP Link.  You can do the routing on a dedicated box in the middle though between the TP link and Modem/gateway in bridge mode.  Turn off the DHCP(4/6) server on the Archer (DHCP forward requests) and do your networking on the box in the middle.  You'd get a better firewall that way too, and logging.

IPV6 was pretty unstable here for quite some time.  Perhaps they are also working on stuff in your area. 

Thank you for reaching out to us on our community forums @user_vevbfg! Are you currently using a rental xFi gateway with bridge mode enabled or is the TP-Link Archer C7 connected to a personal modem?