Exactly what USB dongle is causing the problem?
It is a known fact that poorly shielded USB 3 cables and devises can cause both Wi-Fi and Bluetooth interference and problems.
Have a look > Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support where it say's,
If you're using USB 3 devices with your computer, you can limit their potential to interfere with nearby wireless devices:
- Use a high-quality, shielded USB cable with each USB 3 device.
- Move your USB 3 devices—including any USB hubs—farther away from your wireless devices.
- Connect USB 3 devices to the ports farthest from your computer's Wi-Fi antenna. For example, if your computer has ports on the back, prefer the USB ports closest to the middle. If your computer has ports on the side, prefer the USB ports closest to the front.
- Turn off any USB 3 devices that aren't in use.
Or at > https://www.macobserver.com/tmo/article/usb-3.0-hard-drives-can-cause-wi-fi-interference
USB 3.0 connectors and cables can radiate interference in the 2.4 GHz spectrum that causes Wi-Fi and Bluetooth signals to intermittently drop out when they're near the antennas. For the MacBook Pro and MacBook Air, those antennas happen to be near the screen hinges along the back of the computer.
Because the Mac Mini's wireless card and antenna are in such close proximity to the USB ports. It is imperative that you use thicker better shielded USB 3 cables and that all USB 3 devices are far away from Bluetooth devices and not in-between the Mac Mini and Wi-Fi Router.
