Those speeds are perfectly normal for your setup and using WiFi

Wifi extenders are trash.

If your wifi is already trash, you will only be extending trash.

Model of router will help to determine if it's also the issue as it might be the bottleneck depending on your actual netspeeds you pay for.

I'd still recommend disabling the routers wifi completely and hooking up a wireless access point that has wifi6 capabilities. Your device's might not be able to handle wifi6, but they will still achieve the wifi5 the aps can put out.

I personally use ubiquiti, but tplink works as well. Better signals, strength and stability all around.

That said, wired over wireless any day.

You might want to try an access point. You could use an old router if you have one. At any rate I would run a cable to it instead of relying on wifi.

WiFi extenders are never the answer.