Hmmm managed switch-like.
The link I was referring to was negotiated link i.e. 1000BASE-T full duplex vs. 100BASE-TX full duplex. If all are being set to 100, add devices one by one with known working 1000 base gear first. Clear the DHCP leases & check for auto-negotiate? I feel some device in the loop is forcing the drop to 100...or cables. One by one assessment should do it if you can reset the device to brandnew/default, I assume it has a DHCP page to show you link status etc. Keep us posted. I recently bricked 2 cisco/linksys routers trying to get a wi-fi repeater going for THX's attic computer.