Thanks for your help Peter! 
I had the network operator’s service technician visit me to integrate the HomeWizard P1. If I remember correctly, he said that the P1 on the Landis&Gyr E450 isn’t a P1 port, but rather an M-Bus port.
The HomeWizard P1 can handle it, though. Whether it also works with the Homey Energy Dongle remains to be determined. The only annoying thing is that the HomeWizard P1 frequently fails with Homey, so I ordered the Homey Energy Dongle 
