@RobvdBo 

In some cases it can happen that USB 3.0 devices connected to the router's USB 3.0 port create so much interference at certain frequencies that it will cripple the data troughput on the 2.4 GHz band, especially if the device is connected via an USB extension cable.

Some may suggest to manually switch the 2.4 GHz Wi-Fi to another channel, but I think even if you were operating on the same channels as your neighbor(s) router(s), that shouldn't bring the throughput down to 5 mbps. But perhaps it's still worth a try.

@woozle Thanks for your reply. I have no devices attached to USB 3 and I already tried other channels too. I'm suspecting my neigbors or some rogue device is messing up my 2,4G channel, or a faulty router. But how do I check if the spectrum is clean...?

@RobvdBo 

You could try some of the free Wi-Fi Scanner software that show a graphical representation of the Wi-Fi networks in your neighborhood, like "Acrylic Wi-Fi Home" for Windows PC or "WiFiAnalyzer" app for Android. There is probably similar software available for Mac and iOS.

However, if the interference was something other than Wi-Fi signals, then these software would not show that.

To find out if the problem is specific to your AX50 you would have to test the 2.4 GHz Wi-Fi band with another Wi-Fi router at the same location. 
In case you don't have another router available for testing, but do have a modern smartphone with a 4G data plan and get at least something like 50 Mbps download speed at your location, then just use the Wi-Fi hotspot feature of the smartphone to test the 2.4 GHz Wi-Fi band.