M-bus to Kamstrup 403 not working

Your 403 uses M-Bus (Meter Bus) which is quite different from Modbus.
I’m not familiar with M-Bus, so unfortunately I can’t help. But I do know that’s it’s not
the same thing as Modbus. Perhaps someone versed in M-Bus can help.

Edit - I see that a Modbus module for the 403 is available.
If you have the Modbus module, I might be able to help.

Ref:
https://meters.co.uk/m-bus-vs-modbus-difference/