Netgear R6020 Horrible Wifi issues

Hello I am currently using my old netgear r6020(ac750) running luci openwrt and running this router in client mode to give my desktop wifi and I have sqm installed but idk how to set it up properly and idk how to configure the router for best performance I just want to be able to use the open wrt router to give my desktop wifi and no other devices just my desktop and if we can fix this issue is there anyway to geofilter to get better ping in games?

I have a spectrum internet package for 400 mbps Download and 25 mbps upload

The R6020 has 10/100 Ethernet ports and a 520 MHz single core MIPS CPU. It's not suitable for a 400 Mb service.

I only get 10 mbps download with it am I doing something wrong cuz doesn’t it support up to 100 mbps and the only device connected to secondary router is my desktop