Sounds very frustrating but it’s not surprising the other solar/battery installers don’t want to get involved if it is not there installation. Troubleshooting can take many hours and numerous phone calls/emails dealing with tech support etc. (which they might not be able to offer a solution or get paid.)
This is where LG Chem should be taking the lead and helping guide through the problem solving process with yourself or another solar installer. You have been left in the dark which is very disappointing.
There is a dedicated communications cable (CAT5) between the inverter and the battery which I’m assuming has been checked? If the comms is disrupted or unreliable the battery automatically trips off. Also the battery can trip off under low voltage or low SOC.
Unfortunately if the battery is left off for too long it can ‘die’ and may not be able to be restarted without specialised equipment from LG. How long has the been been turned off?