Yes M1 IPv6 works well for me, but i'm not using an Asus router. I can say that the M1 side works 100%, but i'm not 100% clear on the correct settings for your router. Perhaps someone else is more familiar with your particular router model.
What did you set the WAN connection type to? It should be either "native" or "passthrough".
Does IPv6 show up on the router, what client devices are you using to test and is IPv6 enabled on them, and do you have any other devices (eg additional access point) between your client device and the asus router?
Edit: looking at the guide above i suspect you should try type native rather than passthrough. It should give you a DHCPv6-PD option, which also needs to be on.