I think the difference is the fact that your "WAN - Internet Connection" is not the same as I am using.
Yes, sorry I hadn't gone that far backwards in the chain. I shout have included those details too.
You have a selection using PPP on the IPV6 setup and I do not even have that selection as on my picture of the IPV6 settings.
On this specific router, there is no PPPoE support for IPv6, that's why I am using PPP. I believe other (later) Asus routers do now support PPPoE as a WAN mode for both IPv4
and IPv6.
I am using "Automatic IP" and I think you are using "PPTP" for your "WAN Connection Type" but it is possible that your provider provides a different connection type and I am pretty sure that you are not using "Automatic IP". So those other selections are not available on my setup.
Yes that's almost correct. I'm using PPPoE for "WAN Connection Type" not PPTP although I can and do use VPN too - separately. I use PPPoE
despite me having a Static IPv4 address. This is purely because I am using IPv6 too, but, without the router's ability to support PPPoE for IPv6... Otherwise I could successfully use the Static IP "WAN Connection Type" for both IPv4 and IPv6.
I do have a static IPv6 address (of sorts) too,
but only static as far as a firm, non-variable LAN IPv6 Prefix is concerned (hence a fixed pool of IPv6 addresses from the router) but, my own WAN IPv6 address can change, after a router re-boot (unlike IPv4) but, it changes
only within the relative WAN IPv6 pool itself (if it does). This does NOT affect the LAN IPv6 Prefix and thus the fixed pool of IPv6 addresses provided by the router to change, but the WAN IPv6 address change is enough to make getting DDNS via IPv6 to run properly, a lot more challenging that it otherwise would be...
Moving back to your initial point, yes, there's quite a lot of differences, both visible and knock-on (e.g. IPv6 as you've mentioned) as a result of two different WAN Connection Types: Automatic IP and PPPoE. That means, any chance of us identifying a subtle difference in our same router / same firmware setups as the cause of your IPv6 failures, has now massively diminished, sadly...
The IPV6 DNS servers I am using currently are there because I -sometimes- get a hostname for IPV6 if i use another DNS server with the Google DNS Servers. I say -sometimes- because it never appears if I just use the Google Servers. If I add another server like Quad 9 DNS server then sometimes I get a hostname in the ipv6-test.com, but not all of the time. I have been attempting to find if I can make it a little bit more permanent, but no luck so far. The picture I provided is where it did not get an IPV6 hostname.
I see. However I'm not sure having a hostname for IPv6 is that important is it? It's only relative to your specific device and its absence doesn't affect your score rating on that specific test site at all, or indeed, affect anybody's ability to reach you AFIK. I haven't had a specified IPv6 hostname - by choice. You can try here for some further reference IPv6 Tests:
https://test-ipv6.com &
https://tools.keycdn.com/ipv6-ping & you'll see that having a hostname is not a factor with either.
FWIE It might still be worth giving the 'only 1 DNS provider (IPv4 & IPv6) & a then router re-boot a quick shot and seeing what happens to your IPv6 reliability?