Hello guys.

This is my very fist post in a forum so maybe I don't know all the proper procedures. I am a non native English speaker, so please excuse the hopefully only occasional errors. 
I have seen, that some people already posted about similar issues, however I couldn't find a working solution yet. My problem is the following:

I bought the add in card (archer tx3000e) because I need wifi in my student apartment. Until 3 days ago it worked mostly as expected, just after the boot to windows 10 I always got one disconnect. Back then I thought it is an antenna problem, because after rearranging them everything worked fine until an eventual reboot. This was on an MSI B450m PRO-VDH Plus Mobo and pure windows system.

So three days ago I rebuild my pc with a new mainboard (Gigabyte Aorus Elite X570) using the same Ryzen R5 1600af as before. Additionally I replaced the power supply with a bequiet! 750W gold unit. I installed Windows 10 first on the wrong SSD (M2 SATA) but then I first noticed the problem which is persistant ever since. My PC just can not keep up a stable connection with my wifi network. After the boot it is connected, but as soon as I want to download, steam or access something form/on the internet my connection breaks down. If that happens I still can see my network and all the neighbours networks. After that it randomly reconnects to my network automatically or when I manually initiate a reconnect.

So my fist trouble shooting step was to reinstall windows 10 since it was installed on the wrong drive anyways. Now it is running form the intended Nvme SSD, as it was on the old motherboard. But unfortunate I couldn't get rid of the problem.
Next I changed the slot of the wifi card to the other PCIe x1; the problem persists. Same with the 2nd PCIe x16 slot. My thinking behind that step was, that maybe my CPU, which is not a really good match with a x570 chipset, can not handle all the slots, but since the x16 slot didn't solve my problem, I no longer think that this is the case (plus I think I found the root of the problem, which follows now).

My next step was to use my phone as a hotspot, to see if my hardware was damaged after the rebuild of the system. The connection with the hotspot worked flawlessly. Knowing this, I figured, that maybe my router was doing some weird switching of the wifi channels, while my PC tried to downstream. So I set the channels (from my tablet, because my PC can not connect to the router) for the 2.4 GHz and the 5 GHz to a fixed values, but that didn't solve it. Next I disabled the 2.4 GHz; same as before. Finally I switched on the 2.4 Ghz and disabled the 5 GHz connections. Success! Through the 2.4 GHz connection everything worked. So I set up my system and downloaded the drivers for my Archer TX3000E and installed them.

I figured, maybe now the 5 GHz connection would work, but unfortunately it doesn't. Same issues as before. Since I setup dual boot to Linux (Kubuntu 20.04.1 LTS) I tried it there, but basically it shows the same behavior (Websites load on linux, but larger amounts of data like streaming via youtube doesn't work).

Right now I am running the card on Windows with 2.4 GHz 802.11 b/g, so 5 GHz is basically disabled in the device options. However I wish to run my PC with 5 GHz. One reason is, that my router is too incompetent to intelligently share the bandwidth with other devices in the network, while I am using this setting. So when I download something, everything else dies down. Since I paid for it, it would be nice to be able to use it too :)

It would be great, if someone knows what to do or has any suggestion, how to solve my issue. Also I find it quiet strange, that it worked on my system before (except the one obligatory reconnect) even though I just changed my mainboard and power supply and added the M2 SATA SSD.

Additional system info:

CPU - Ryzen 5 1600 af at stock clocks

GPU - XFX Radeon 5600 xt
Mobo - Gigabyte Aorus x570 elite

RAM - Corsair Vengeance LPX DDR4 @ 3000 MHz and stock timings

P. unit - bequiet! Straught Power 11 7500 Watt 80 Gold

Storage - 2 SSDs. One Nvme one SATA (forgot the exact models)

Wifi Card - Archer TX3000E

Operationg system info:

Windows 10 Pro (activated) Build 19042.630 (highest automatic patch) - all drivers checked and installed

Kubuntu Linux 20.04.1 LTS with only automatic 3rd party driver install while setup

Router info:

Connect Boxmanufactured by COMPAL (I guess the model number is CH7465LG-LC and S/N: DEAP706800BA)

Thanks for your help :)