A while ago, I mentioned that I was setting up IPv6 for Heimkoma Services's network here. I had previously set it up with Hurricane Electric's Tunnelbroker program where one could run IPv6 via a tunnel on whatever endpoint one has.
One day, I decided I needed to plug into the modem directly because I was troubleshooting random slowness and straight-up failures of services at the time. I happened to notice during troubleshooting that TWC had enabled IPv6.
Checking Test-IPv6 confirmed that I had full IPv6 working whilst connected to the modem directly.
Let's try plugging in the router and getting it to work natively! Almost immediately, I encountered issues in getting IPv6 to work properly with the router.
Turns out it was a very simple issue. I had forgotten to remove the Hurricane Electric IPv6 tunnel! I'd changed the IPv6 screen to use DHCPv6-PD, with radvd and dhcp6s set to enabled and not using custom configurations.
But I'd forgotten to remove that blasted Hurricane Electric IPv6 config. So the router was confused as to which one to use because both were valid configurations. Once I'd cleared out the Hurricane Electric tunnel configuration and then rebooting the router, everything started working perfectly.
Just as if I was plugged directly into the modem. Just as it should be.
It Just Works.