Thank you very much for all your answers. I've now successfully installed the new thermostat with the suggestions from @Stuckinarut.
So the B cable from the old honeywell box went to the NO on the meross as suggested, while the A was completely left out as it seems to be a spare Live from...
Hi, I have the Meross thermostat which I used in my old house without problems by connecting L, N and NO.
However the new house currently has a Honeywell T6 thermostat in the corridor and a Honeywell unit in the closet upstairs (I'm assuming there used to be a tank/system boiler previously)...