I bought a LG G6 H873 from Ebay. You never REALLY know what you are getting with these. It was “new” and “sealed” yet the included manual was in Chinese. I checked the IMEI by visiting 5 or 6 mine fields of ads and malware before getting a clean bill of health. I used the phone dialer and typed *#546368#*873# to get to the secret menu to view all the actual battery situation after I fully charged the battery. That looked good. The only problem was I was stuck on Android 7.0 and the standard method of checking for updates via the Settings said I was on the latest version. This was a problem.
I tried using LG Bridge to update to Android 9.0. It’s a bit of a disaster of a program in itself. Once I realized that you don’t actually want to connect (their definition of connect means to connect through wi-fi), but instead you want to go straight to Software Update to use the USB connection. It seemed like all was well, but after a 3GB file downloaded, I was told my phone was not compatible with LG Bridge. This turned out to be a lie (or maybe just incorrect).
I went down a rabbit hole of disabling antivirus so I could use Uppercut. Uppercut said my model of phone was “unknown”. The fixes for that led to fixes for that that led to fixes for that. It wasn’t pretty. I read a forum post that said that users of the H873 are just plain stuck and there’s nothing I could do about it (despite many users claiming the opposite). I lost track of all the drivers and tools I installed, but I definitely had LG UP 1.4 (which I believe is LGUP for Store, LG Mobile Driver, LG AirDrive, and LG Bridge installed.
I then read that I needed to enable USB Debug mode by enabling Developer Options. This is done by finding Build Number in Settings > About Phone > Software and tapping on it 7 times. I tried to enable USB Debugging but it was already enabled. I found that interesting. ( I did have the phone connected to my computer while doing this.)
After all of this guessing, I tried LG Bridge again and it connected. In about 15 minutes I had Android 9.0 Pie on my LG G6 H873.
I wish I could narrow down the exact specifics, but I suspect LG Bridge failed the first time because of either a lack of the correct drivers of the fact that I did not have Developer Mode engaged.