AppleTV 4K continually reboots

Our AppleTV 4K continually reboots. The Apple logo appears for about 1 second and then it restarts. Maybe it did an auto-update that didn't go down well.

So far I've tried without success:

1/ recharged the remote and tried to restart it with the Screen/volume down key press

2/ remove the power plufg or 5 seconds and 5 seconds, both repeatedly.

3/ tried a different HDMI cable.

Any ideas?