I see it says “M-bus” on the PCB, M-bus it different to Modbus.
Mbus is not read like Modbus, with Mbus the whole data set is broadcast rather than read by register addresses. It is usually a different/higher voltage (48v?) than Modbus as the meter is powered via the rs485 rather than being a “2-wire” comms only connection (half-duplex).
How did you get the “Modbus” document? That’s definitely Modbus (I think), so I questio whether it is right for that device or if the PCB has a typo?
edit - The sales page says M-Bus too
edit2 - see MBUS Meter Reader, Sontex 531 Heat Meter for info on OEM Mbus reader (if it does turnout to be Mbus).
(also https://github.com/openenergymonitor/HeatpumpMonitor/tree/master/Firmware/Arduino/MBUS_Reader and the Heatpump - #2 by TrystanLea thread too)