Did some long term traces over the weekend. LAN looks good now. My ISP tears down the PPPoE session every 24 hours (via PPP-LCP Termination Request 5 and then via a PPPoE Active Discovery Terminate PADT). And my ISP hands out IPv6 prefixes not statically but dynamically. That works on the LAN side just perfect. The previous IPv6 prefix gets invalidated – not exactly as described in RFC 9096 – but my macOS and Ubuntu computers mark the old prefix a deprecated. Great.
However, the WAN side is still a mess. And I do not find the culprit. My GL-iNet router is sending a ICMPv6 Router Solicitation nearly every hour, not within PPPoE. And indeed, I set-up another router in-front with PPPoE pass-through enabled, and the GL.iNet assigns an IPv6 address to its eth0. I can even ping and the GL.iNet from that net = outside. I can even access it (SSH, Web, …). That must be fixed before going live with that new native mode.
Furthermore, the mentioned tear down – on the WAN side – does not work really. It is more luck or better a-try-on-the-second-run that my GL.iNet copes with that tear down and gets a new IPv6 prefix. In any case, my GL.iNet looses a lot of time – is disconnected much more than needed – because it does not cope with that tear-down situation correctly. That should be re-visited when field testing with various Internet Service Providers (ISP). As ISP, I am using 1&1 in Germany. Should be the same for Telekom Deutschland – however, they tear down not after 1 but 180 days. A bit difficult to test right now.