ISPs cannot "force" a switch to IPv6. If they could we'd be on 90% adoption now.
Maybe you need to troubleshoot more and confirm it's actually IPv6 and not just your router failing to e.g. establish a PPPoE session for your IPv4 connection.
Buddy idk what to tell you. One day ai came home from work, nothing could connect out, all my devices had ipv6 addresses and my router (which I rent from my ISP) was set such that every 4/6 option was 6. I don't know why you think the poorly regulated almost-monopolies can't do whatever they want to the router they have remote access to.
Maybe you need to troubleshoot more and confirm it's actually IPv6 and not just your router failing to e.g. establish a PPPoE session for your IPv4 connection.