I am using ASUSWRT Merlin on both my RT-AX68U (3004.388.4) and RT-AC1900 (386.12) routers.
I use Starlink as my ISP. Starlink provides me a public IPV6 and private IPV4. The DDNS feature provides an option to enable IPV6. The IPV6 works by adding records for both my IPV4 and IPV6 addresses...