@Robert-Jan,

I would just double-check the Amp is definitely plugged into HDMI-3 on the TV and not the nearby HDMI-2 and perhaps try the ‘TV Setup’ option in the Amps ‘room’ settings in the Sonos App ‘Settings/System’.

If available, you could maybe also try another TV, but if the issues persist, then submit a System diagnostic report and note it’s reference and contact Sonos Support via this LINK to see if the diagnostic may reveal anything.

I’d also try unplugging the TV from the wall for five minutes, before plugging it back in, forcing it to fully reboot the OS it is running, something that can’t be done normally with either the remote, or the power button. 

We cannot rule out hardware failure. Power outages are often accompanied by electrical transients that damage equipment. As the transient cascades through the equipment, it is very difficult to predict the failures, if any.

Sometimes the startup sequence is involved. I recommend powering down everything then disconnect all of the HDMI cables from the TV. Next, restart the TV and AMP, then install the HDMI cable between AMP and the TV and verify that the CEC link is operating. AMP will usually acknowledge the connection, I have no experience with your TV. If other HDMI devices are usually connected to the TV, connect them now.

I Tried all of those things. how to I “verify that the CEC link is operating”? While setting everything up I cant go to the next screen in de Sonos app when the HDMI cable is not connected or not in de ARC port.

Only when it is correctly connected It will let me go to the next screen, but than is still says "CEC not connected”

I shut down both devices for 15 minutes (no powercables connected) and reset both devices, still nothing :(

Is the Amp connected to the right HDMI port? It should be HDMI 3: https://lidl-nl.inxshare.com/55%20VLX%20707%20LDL%20-%20Manual%20NL.pdf

It’s possible that there is a TV CEC enable option that was reset because of the power failure. I’m not familiar with GRUNDIG TV’s. Some manufacturers have a branded name for “CEC”. For example SAMSUNG calls it ”Anynet+”.

I think i got those covered