Not appearing in the bluetooth device

It was all fine, and one day suddenly it will not able to connect wirelessly through bluetooth.

Trying reset the pairing process by disconnect the keyboard, however, since then the keyboard is not discoverable as a bluetooth device, whether one this or other Mac.

However, connecting directly with lightning cable allow it to be able to use the keyboard.

Just that it still not appear in the bluetooth device no matter connecting with cable or not.

Tried restart both Mac and keyboard.

So currently I've no way, but connect the keyboard with lightning cable whenever I need to use it, and it become troublesome and occupy one of my port.

Anyone solution? Appreciate the help.