Depeding on how advanced control you want, modbus over RS485 is the way to go (that's how i was planning to do it before i went for a sunsynk inverter due to availability mostly) or you can just configure one fixed time in the inverter
https://watts247.com/manuals/gw/GrowattModBusProtocol.pdf...
I know it is a bit late probably, but it is a standard cisco console cable. You can buy those with DB9 - RS232 plug or more modern version with USB plug and integrated usb-232 converter. Ebay or amazon or anywhere has 100's of those.
On some variants they use RJ11 plug with different pinout though.
Is that price including shpping and taxes?
I've tried to get batteries from china twice this year and both times i've wasted 3 months waiting (luckily i got a refund).