This problem is caused by bad software in an EEPROM chip on the mainboard (IC205) and can be repaired easily. If you flash an image of this software on this chip, your monitor will come back to life again. I did this repair for about 5 times and all attempts were successful.
If your LG 31MU97-B monitor is completely dead and you are not living in The Netherlands, please send your mainboard to me. I can fix this issue and send the mainboard back to you. If you live in The Netherlands, you can make an appointment via my website mentioned below.
Kind regards,
Erwin Aarnoudse[br]
Monitorkapot.nl[br]
The Netherlands[br]
https://www.monitorkapot.nl