e7240 lan issue while docked

Hi,

I've been experiencing this issue for some time now, and haven't found any permanent solution. The lan port on laptop itself works fine, once docked the link lights go down which indicates that the connection has been relayed to docking station lan port which doesnt light up when cable plugged in. Things that I already tried are: Bios upgrade, downgrade, multiple different lan drivers, different power adapters for docking, reinstalling windows, hardware diagnostics, running without battery, trying 15+ different docking stations. What really confuses me is that theres software called Dell power manager, and when I changed the default setting from Adaptive to Standart it did work for some time with any docking station, when it stops it would only work from switches/routers that have PoE ports. All power adapters that I used for testing are 90W. I'm out of ideas here, any help would be appreciated.