What LAN cable to use for WiFi extender

I'm helping fix internet issues for a local organization. Their router is pretty far away from where they use the WiFi so they set up a WiFi extender. The problem is they didn't hardwire the extender to the router and it cuts out the internet a lot. This could be due to it trying to reach the router 50' away through 2 concrete walls.

I recommend to them to hardwire the extender to the router to see if that helps. If the cable doesn't help I suggested they change the WiFi extender to Google WiFi.

However there are a lot of different LAN cables. CAT5, CAT6, CAT7? What would be the ideal cable for running a 70' cable from a router to an extender?

If it matters the router is a Cisco DPC3939B and they have Comcast business internet.

Thanks.