It's a Swisscom FTTP fiber connection. The R3 was connected downstream via a Swisscom Internet Box, since the R3 cannot handle Swisscom's VLAN 10 directly on the WAN port.
The cable and ports are completely fine, verified with other devices.
For the pinhole reset, I held it in for over 30 seconds multiple times.
As mentioned, the unit is completely bricked: the power LED just blinks indefinitely, there is zero RGB lighting, the Swisscom box detects it as offline/powered off, and it doesn't respond to any reset or recovery mode. It ran fine for 2 years and suddenly died.