I've disabled IPv6 on my ASUS RT-AC68U but devices on the lan still seem to have IPv6 addresses from the DHCP server. Is it actually possible to disable IPv6 on the LAN side? I'm suspecting that the router's IPv6 settings page refers to the WAN connection
In the IPv6 setup page at the bottom, there is a section called Auto Configuration Setting, Enable router Advertisement is normally set to enable, have you tried disabled? You might need to reboot if it don't do itself after changing and applying.
Is there a reason for disabling? Since it is a technology of the future I would try and fix the problem rather than band-aiding it ie disabling.
I have a device here where ipv6 cannot be disabled on the newer firmwares. I would not advise downgrading firmware (as is often the "fix") since this places your network at a great risk, firmware upgrades often resolve critical security risks and loopholes.