@Areks 

Disable QoS.

Change the Channel Width for the 5GHz-2 band to 160 Mhz and test:

You may check this story for additional details on 160Mhz channel width setting.

  @Areks

To avoid any other conditions that are limiting your bandwidth - disable QoS on your AX90.

Here's your bottleneck according to the screenshots you posted:

If your Dell Precision 3561 wireless adapter is Intel Wi-Fi 6 AX201, you should be able to achieve 1200mbps on the screenshot above ( 5 GHz-2: 1201 Mbps (802.11ax) )

If you try the same with 5 GHz-1 band which is 4804 Mbps (802.11ax, HE160), you should be able to achieve full ISP speeds.

Here are the AX90 technical specs:

Since the nature of the wireless ethernet connection is half-duplex, you won't be able to utilize your full ISP speed on 5 GHz-2 band.

  @Areks 

It's easy actually - just use different SSIDs for different wireless bands. Thus you'll know to which wireless band of your AX90 you're connected at any time.

When you need a wireless client device to use your full wireless radio bandwidth connect it to the 5GHz-1 band SSID. If a wireless client device is not so bandwidth demanding, you can connect it to 5GHz-2 band SSID, to offload the 5GHz-1 band.