@Froggy650 

Hi,

Is that USB disk a thumb drive that plugs directly into the router's USB port or is it a disk that is connected via an extension cable?

In case the USB disk is connected via a cable, then that cable must be insufficiently shielded. See if you can replace the cable with another one.

  @woozle Hi, yes, it's a M.2 SSD connected via the USB converting cable. I will try with a USB stick.

  The problem happens with any USB stick or an external USB disk. I'm thinking to return the device to the shop. Or is there a solution? I worry it's a hardware issue... 

  @Froggy650 

The only "solution", if you want to call it that, would be to limit the USB disk to USB 2.0 mode.

I have seen routers that have an option in the settings menu of the router to limit the USB port to USB 2.0 (often called something like "USB compatibility mode"), but the AX55 Pro doesn't seem to have it.

So, with the AX55 Pro the only way to limit the connection to USB 2.0 would be to use an USB 2.0 cable to connect the USB disk.

For explanation, the issue of USB interfering with 2.4 GHz Wi-Fi (and other stuff that works in the 2.4 Ghz range, like wireless computer mice) only happens when using USB 3.x (aka "SuperSpeed") mode.