Can't Disable IPv6

Does anyone know how to disable IPv6? I am running on a Raspberry Pi using the image provided by Raspberry Pi Imager

I have tried:
editing sysctl.conf - readonly filesystem error
Network settings in the web ui - doesn’t do anything
nmcli connection modify <connection_name> ipv6.method ignore - got unknown command error, not sure I entered it properly, or in the correct location. But the post I got it from (Totally disable ipv6 - #5 by mendoza.fidel7) seemed to imply that it wouldn’t work
Taking the sd card out and mounting it on another computer (running linux) - It mounted 3 partitions that did not seem to have any files that would be helpful. It also gave an error which I believe is because the primary partition uses a filesystem I am not familiar with and do not know how to access

Beware that Matter require a working IPv6 network to work.

And I guess a disabling in the router would be the best solution.
If your router is not configurable in that way, then replace the router or put a configurable router behind the non-configurable.