@Geo57 

How do you know that the PC connects to the guest network via Ethernet?

When connected via Ethernet, there is technically no network name.  Windows will apply a name to the connection, but that name has no technical impact.

  @Geo57 

Hi;

I think I solved this by editing the network profile name in the policy editor. Before I did this there were two network pofile names in the registry: under \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\NetworkList\Profiles\. I edited the guest network name with no effect. Next I deleted both profiles, rebooted and reconnected to the network and internet. The new network name was "Network 2". I edited this name to the deco name I originally had, rebooted and the name stuck. All is good now.

This is how I think it all happened: At one point I was connected as always. I inserted a usb wifi network controller and connected to the wifi from the deco router: somehow it was the guest network. Removed the usb wifi controller and rebooted. I was stuck with the guest network as the wired network name. I think it was solved. It was all in the registry.

  @Geo57 I believe the name showed by Windows for Ethernet connections does not have any impact.