Disable ipv6

What is it that's you're trying to do? Since you're wanting to disable it for your internal network, it might be an option to do so via the individual operating systems, although I would definitely recommend researching the device and operating system to evaluate the impact it would have if you decide to go that route. 

under DHCP i can't disable the DHCPv6, when I unselect the option and save the toggle come back.

I can't disable ipv6 on every device in the network, it's insane.

This forum thread is a bit older and for a different modem, but the instructions in it are still accurate : https://comca.st/37LhiIb . Is that what you're trying?