So the Ethernet connection between the 2 routers goes through a wiring panel? Did you make sure the wiring are connected to the right "pair" as they were?
The reason I ask is, about 18 years ago, I bought a cable from ebay and the seller crimp the wiring pair wrong, so instead of the 1-2, 3-6, 4-5, 7-8 pairs end up being 1-2, 3-4, 5-6, 7-8, and one pair is not twisted as expected. My network would go 10Mbps but not 100Mbps. I research online and found that, and then re-crimp it myself and the problem went away.
If you can borrow an ethernet switch to test, you can use that to rule out the router or the cable. Narrow it down to which component and go from there.