Hello,
today I tried to connect the ether 5 port on my 2011 router.
The status says that there is no link - the cable is ok, because on ether 4 port it works.
When I reduce the speed to 10Mbit the ether 5 port works but on 100 or 1000Mbit it gets no link. 
What can I do?
To make sure I have this correct in my mind:
With the same device on the other end of the cable, you move it to port 4 changing nothing else, and it just works? Move it back to 5, still changing nothing, and it only works if port 5 is configured to 10Mbps?
/interface ethernet print detail where default-name=ether4
/interface ethernet print detail where default-name=ether5
Double-check every last setting in the two ethernet interface configurations. If they’re identical (except for name and mac address), then it sounds like a hardware fault in the 2011.
Yes, the same device was recognized on port 4 as gigabit device.
The interface settings were the same - I double-checked it.
Totally strange - I also tried two other cables - same problem.
Now, after some shaking, it works - maybe there was some dirt in the jack!?
I am happy that the hardware is not defect. 