This guy seems to have figured it out:
https://wheresmykeyboard.com/2017/08/bluetooth-keyboard-asking-passcode-not-giving-pin-type/
Aparently you can use the Bluetooth and Devices menu:
- Activate bluetooth pairing / bluetooth on your keyboard (so it is detectable)
- Press Add Bluetooth or other device
- Double click to connect
- Your receive an enter pin dialogue, on your computer type "0000" and press connect
- While it says connect, press "0000"+enter on your keyboard
You may need to first remove the pairing in the Bluetooth and Devices menu to get this to work by pressing the device text and then pressing the "Remove device" button
The keyboard seems to time out after a period of inactivity, but it seems that turning it on and off solves the issue sometimes (or repeating the connect process...)
Good luck!
Tested on a linocell iPad Mini 1/2/3 Keyboard