@Phil Bennett , @John Vreeland, @RasPad Support
I've tested following:
PI on ethernet cable to switch:
-Speed autonegotiate/fixed 10-100-1000 HD FD
Same I tried with both the interlink cables and a F-F cable connector: everything works
Then I tried using various combinations of interlink and PCB (I have 2 of each) en all the above mentioned link settings in my switch for the specific port.
With the SunFounder PCB in between, NOTHING WORKS on eth0.
I've traced out the PCB and I suspect there is a MDI-MDIX issue because the pins on the inside and outside connectors are not 1:1 (pin 1 inside = pin 1 outside ... pin 8 = pin 8).
Something is definitely wrong in the PCB traces. Either it's a pin-to-pin connection error or maybe an impedance/conductance issue making the pcb pasthrough incompatible with (gigabit) ethernet.
sorry for my late reply.
I will see in the coming days if I can beep out the connectors on the inside and outside of the pcb.
I also reported my findings to RasPad customer support. Because they already supplied me with second PCB but that did not resolve the ethernet issues