Cannot undo "forget Bluetooth device"
I faced a similar situation. Following steps worked for me. Some of the steps might be redudent.

1) restart your phone

2) reset network settings (Setting> back and restore )

3) change the phone bluetooth name. (I assume this is the solution)

4) set the bluetooth device in pairing mode (long press the power key).

And it worked for me. Probably, executing only step 3 would be sufficient.

I hope I saved your precious 30 minutes:)