Yes, the FW 386.4 Beta 2 was the last time that things related to IPv6 DDNS worked (of a fashion). It seems that everyone; All IPv6 users / IPv6 DDNS users / RMerlin / You / Me / A.N.Other etc will need to wait, until Asus move their own 386.5 Betas to Official Releases and then... we can all see what's been fixed / wait for the Asuswrt-Merlin 386.5 FW release(s) & then try again....
I have posted before, that I am using No-IP DDNS (as they do support IPv6 DDNS) as well as Asus DDNS and with the official Asuswrt-Merlin 386.4 FW release (i.e. I'm not using any previous Beta), I can quite easily & successfully,
nslookup both the IPv4 Asus DDNS and IPv4 No-IP DDNS and... the IPv6 No-IP DDNS too (via
nslookup -query=AAAA $hostname). The No-IP IPv6 DDNS is NOT being
updated from the router, which is correct, as you've read in several other posts (removal of IPv6 DDNS functionality after Beta2 etc). In the interim, IF it changes, you must manually update the IPv6 / AAAA address on the NO-IP website yourself... until... maybe... the 386.5 'fixes' arrive for the acknowledged IPv6 DDNS issues. However, it's still a valid IPv6 DDNS address & it works. It's nowhere near perfect / as it should be / as you want it to be
@Yota but, it
might be worth a look for you in the interim, instead of going back to the Beta2 release, until the 386.5 'fixes' finally arrive?