The Mac Mini's have always had poor Wi-Fi and Bluetooth reception when compared to an iMac or iOS device.
Poorly shielded drive cables, display cables and a long laundry list of other electronic devices can seriously interfere with the Mac Mini's Wi-Fi and Bluetooth reception. To make matters worse, the wireless card is built in the back, the antenna is built in the bottom and easily blocked by shelving or by stacking it with other devices.
see > https://support.apple.com/en-us/HT201542
At this point I can only suggest rearranging your setup with the Mac Mini away from other devices and/or maybe standing on it's side to get the antenna more up and out in the open.