cabled

So I have moved to a place where the modem is located away from my room. It's only a few meters away, but I can't run a cable through the wall. I have bought an extender and I have connected a cable from the extender to my pc. This is the same setup I had at home and it worked perfectly, but...

I have been having internet problems since we moved into the house and got a new wifi provider that should provide 750 mbps, but I seem to only be getting 12 mb/s max (sometimes 12.4 or 12.5 never more). Any ideas on how to fix this? motherboard; gigabyte x470 aorus ultra gaming. Pictures: