Grummerstrawn Welcome to the UPC Community!
The signal quality is fine. Also we can’t see any interruptions on the network side.
What we also see is that currently 9 devices are connected via 2.4GHz WIFI and none via 5GHz or LAN.
You could now connect the devices that are closer to the modem with the 5GHz WIFI and all the others with the 2.4GHz. We also recommend you to connect the important devices via LAN cable. What we could do from our side would be the change to IPv4.