Should I use cat6, 7, 8 (or something else) ethernet cable?

I’m trying to turn an old router into a WiFi extender. I’ve been told an Ethernet cable is the way to do that.

But when I went to look on amazon, loads of different options came up. I didn’t even know cat 7 or 8 was a thing. And to be honest I don’t understand what the difference between cat 6 and 6a is.

So would it make a difference if I got cat 6 vs cat 8 cable or anything else I might be forgetting?

if it matters, I have dsl for internet