The following information comes from Sonnet's website.
The steel mounting case effectively shields the Wi-Fi and Bluetooth antenna from effective use, and, in fact, YOU MUST TURN OFF wi-fi in the xMac mini Server TB3 edition or it may intefere with Thunderbolt functionality.
For Wi-Fi, some customers have used the Edimax EW-7822ULC external USB 2.0 wi-fi adapter; or the Edimax EW-7822UTC or Edimax EW-7822UAC external USB 3.0 wi-fi adapters.
For Blutooth, some customers have used the Startech USBBT1EDR4 Bluetooth 4.0 Adapter.
To use an external Bluetooth 4.0 adapter on macOS:
1. Open Bluetooth System Preferences
2. Unpair all bluetooth devices from the internal bluetooth.
3. Check "Show Bluetooth in menu bar"
4. Option-click Bluetooth in menu bar, and note the internal Bluetooth address
5. Turn Bluetooth off and wait 5 seconds
6. Plug in the Bluetooth adapter and wait 5 seconds
7. Turn Bluetooth on.
8. Option-click Bluetooth in menu bar to comfirm that the Bluetooth address has changed to the external Bluetooth adapter
9. Pair your Bluetooth devices
10. To make the selection permanent, in Terminal, enter sudo nvram bluetoothHostControllerSwitchBehavior=always