Orbi960 Wired Backhaul link Speed capped at 100 Mbps

I agree, bad wiring will cause the connection speed to drop to 100 (or nothing if it is really bad).  But, that said, why you see a 500Mbps connection through a device claiming 100Mbps max is surprising, right?

Here's a super easy test for you to eliminate the cabling variable: 

Take your satellite and place it 3-6 feet from the RBR and connect it with a short CAT5e or CAT6 patch cable and check the reported link speed.  If you get the 2500 or 1000 that you should be able to, then you know the wiring is faulty.  If you do not, then the problem is something else - perhaps a port on one of the devices or the software reporting an incorrect link speed (which I have not seen happen on my system).

Alternatively, if you have two Sats (I don't know what you have on the other port that is hitting 2500), swap them and see if the problem follows the Sat or the cable.

Do whichever is easier and let us know what happens.