Upgrade wifi from IPv4 to v6?

Hi, In 2017 I signed up to Yahoo BB with IPv4 PPPoE. It’s a softbank membership. I have the classic NTT home gateway with a wired buffalo router in AP mode.

I have good wifi speeds in the day but very slow speeds in the evening, would the logical upgrade be to IPv6 and if so, how would I go about that? I have searched but results seem confusing.

Also, I cannot get wifi on the top floor (concrete building) extender doesn’t quite reach. Would a mesh network be an option instead of the buffalo router?

Any help other suggestions would be much appreciated.

Thanks