Did you reboot the router after setting it to Native IPv6 mode?
Did you fully power down the router and the ISP's equipment for a few minutes before powering everything up again?
Yes - rebooted the router.
The only "ISP Equipment" is the ONT on the wall, which isn't tied to any one particular ISP, but yes that was also powered off and on again.
There's no Modem or anything - ONT straight to the ASUS WAN via ethernet cable.