I have an R7 3700X, CPU usage is fine. That is the problem though, particularly for smaller games: a lot of the "download" process is decompression rather than actual transfer, so the overall "speed" reported is way below what maxing the connection would be.Check your CPU usage, they are decompressing at the same time as downloading and that can be the bottleneck sometimes.
Keep it, switch Wifi off and wire in a dedicated Access Point such as a UniFi one and put it on your upstairs landing. Let the UniFi handle Wifi and the BT hub do the modem and routing part.
You don’t need it to be modem only, you’re just disabling the WiFi on it and using it on a product which can be placed better and as such is more than likely to give way better coverage.
It'll work on the wall but you'll get much better coverage with it on a ceiling.