@Clive_A 

Thanks - and sorry for not having been precise!

So, my post is already a couple of days old - and, fortunately, all services are back up since this morning.

What did I do to resolve? Of course I checked all DNS entries, changed to different providers and then returned to my original values, i.e., 1.1.1.1 and 1.0.0.1. Or the router address 192.168.0.1. Success? No. Have restarted the router several times over the past days: no improvement. Also restarted switches, no impact. Yesterday evening I was so frustrated that I simply took everything offline/powered off and restarted/powered back on this morning. It worked, everyting came back. I however saw in the Omada App that (some) devices were re-adopted.Perhaps something went wrong in the communication between the router and the rest of the infrastructure. 

On the errors I have seen: Apple App store worked, Samsung/Android update services didn't. Windows update? No. Smart home solutions such as Philips HUE? No service. Miele Smart Home? No. Garden watering system (Gardena)? Yes. Connectivity to VoIP provider? No. LG smart devices? Yes. "No" means either an error message from the App such as "No connection/service unreachable" or just an animation /(running circles, ...). Interestingly, all smart home services were reachable from outside. But not if connected to the internal LAN/WLAN.

OPNsense: Using OPNsene as transparent bridge beween the router and the rest of the network. No routing or other functionality. Just sitting there and scanning traffic. The update function did not work, just an endless animation of a rotating circle.

Web pages: some public pages opened immediately, others presented content veeeeery slowly, came up with a "time out" or simply an empty page.

The only constant element over these past days was that those services which worked, always performed and those which didn' t open, remained unfunctional.

 So, I assume that was a hick-up somewhere in the network causing this behaviour.