rowesley

Hi Horse,
the inverter should work and no need for an external charger. Are you completely off-grid or do you have also some AC input from a provider just to test? The AC input should be somewhere in the range of 170V-250V ac 50Hz. The generator output should be also standardized 230Vac 50Hz. You have to measure at least the generator output voltage with an AC voltmeter prior to connect it to the inverter. Also the ON power button below the inverter case should be turned on.

OK, for 5W in minus the quiet is more interesting. However, if you want to get off the error 14, you have to trick the inverter motherboard. This is easy: use a CMOS type 555 as square wave oscillator and set the output frequency above the limit of displaying the error 14. Connect the oscillator output instead of fan RPM output. This will not turn off the inverter anymore if fan becomes defective.

Perhaps: a) check if indeed A and B are tied together in the connector, b) use a slave MODBUS software on PC, and an RS485/USB converter.  See what is sending the inverter using a PC software like https://www.modbustools.com/modbus_slave.html
A BMS protocol configuration implemented in some inverters is described here: https://workdrive.zohopublic.com.cn/file/vrz5k03066cc0152f4740b9bfff16dd8afe47
or, do nothing and let BMS to do it's job. 🙃

If do you have a RJ45 mother connector named RS232 on the bottom of the inverter near the RS485 RJ45 cable used for LiFePo4 BMS control,  then connection you need is almost sure the next one, just build your cable but DO NOT connect the VCC from RJ45 to pin 9 of DB9.
DB9      RJ45
9   Vcc    5
2   Tx      1
3   Rx     2
5  GND  8
With the cable above you should measure the following voltages: GND to Vcc = +13V, GND to TX = -12.6V, GND to Rx = 0V. Do not connect the Vcc wire in DB9 connector.
Connect the manufactured cable to a computer  by using an RS232 to USB adapter (a laptop running on batteries without being supplied with any adapter if you are afraid about AC currents flowing from inverter to laptop) and run Modbuspoll ( you can download an evaluation copy). I have the meaning of all important registers you will read with Modbuspoll , but I can post this info only on Monday.
Ok attached the internal registers of Vevor 3500W MODBUS inverter. Yours should be more or less the same.
communication parameteters 9600,8n1, reading holding registers ( address 03) useful data starting from addr 200 to 323. Between addresses 247 to 292 all registers values are 0.