Old SMO 40 - where are the MODBUS connections?

Having got very excited about connecting my SMO 40 to HA using MODBUS using a Pi Zero W with RS485 CAN HAT, I took off the cover of the SMO 40 and was faced with a rather intimidating collection of boards and wires:

I am completely new to MODBUS. I can see from the HAT that there are 4 connectors called H, L, B and A. Is H Vcc and L the GND?

Does anyone know of a step by step idiots’ guide to setting up the hardware and then the software in the Pi please? I have used this link to get SocketCAN working on the Pi.
I looked at the link provided by HA at OpenHAB. I was a bit lost when it started talking about “things”. And then in the section for Pi’s I see I have to load nibgw.
Do I use the serial or UDP option (or are they the same?)
Is configuring my HA to use the data from the Pi just a case of adding it’s IP address to the device configuration in HA?
All help gratefully received!