Youtube slow on ios devices with c6300 gateway.

Already rebooted, changed the cox dns to opendns and it solved the issue temporarily. However, how can i turn off ipv6 because that is what causes the slowdowns. My iphone doesn't have an option to turn it off.

Update: I've changed the dns to cloudfare dns but left the teritary dns to the cox provided one. Youtube takes 5-10 seconds to load but no buffering.