Device visible in HA but not in Router

My HA logbook shows the MAC address of a device, which apparently is detected by my Asus Router. It connects for 5 minutes, then disconnects for 10 minutes, reconnects and so on.

Window2
However, I can not see this Mac Address in my Router interface (which shows the connected devices). So I am wondering where this device is coming from?

Regards
Djiest

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Update: I have found the device (cable modem) but still wondering why it shows up at these intervals