I bought a Mega 2560 from RobotDyn.com for a project and found the Ethernet port wasn’t working or was very unreliable. The shield (active PoE in my case), which comes soldered on the board wasn’t being recognized.
After hours of debugging, I found a solution in the Amazon reviews.
In the back, there are 4 solder jumpers, with the D7 one soldered. You need to desolder that jumper, and ta-da! Working like a charm.