"At the moment I'm testing how my new router works with my old Belkin USB adapter that only handles 50 Mbps but everything seems good so far..."
Doesn't this tell you your new adapter is the issue? Try another make or model.
I will try to explain my problem as best as I can. I'm connected to internet via USB adapter on my PC. A few days ago I received a new router from Telecom Speedport Smart (and a new contract for - 100 Mbps) and before that I upgraded my 10 years old Belkin USB adapter f6d4050 v1 to a new Edimax AC 1750 adapter so I can handle the new speeds. At first my connection was normal and then after an hour or two of using the internet my connection would just become so slow I couldn't open a single web site. I had to reset my router and PC to make it work again but this fix never lasted. The problem repeated itself every day. My PC shows that I am normally connected and all the lamps on the router seem fine. I don't get any errors.
The wireless connection on other devices works perfectly, I even tried connecting my PC to my router with a cable and it worked just fine. I also tried resetting all network settings and reinstalling the drivers but that didn't work.
I also have to point out that my Edimax USB adapter worked just fine with the old router Fritz!Box 7312 (16 Mbps speed contract).
At the moment I'm testing how my new router works with my old Belkin USB adapter that only handles 50 Mbps but everything seems good so far...
Oh and Telecom did run some test, my wiring and router are working as intended.
I assume the problem lies with my new adapter or some settings but I'm not able to diagnose the the problem.
If you need any other information I will be happy to provide it. Any suggestions how to fix this problem are appreciated!
New router - Telekom Speedport Smart 4029491802/J9
Old Router - FRITZ!Box 7312
Most of the time there is one laptop (Asus K555), and 2 cell phones (Galaxy S7) connected to my router but I didn't notice a difference having those devices connected or not connected
New USB Adapter - Edimax AC 1750 Dual-Band
Old USB Adapter - Belkin f6d4050 v1
OS - Windows 10 Pro, Version: 10.0.16299 Build 16299
Motherboard: Gigabyte 970A-UD3P