New BGW320-505, IP Passthrough, Asus RT-AX55 router, CAT5 ethernet cable - HEADS UP!

Just a heads up, or maybe a "duhhh". Yesterday we said goodbye to our old 100 MB cable service and hello to ATT 1 GB fiber. We first checked speed via a router wifi connection, and were shocked at how slow it was. We then checked the BGW320-505 wifi speed, and it was as expected. To our surprise, the BGW320-505 reported only a 100 MB link between its 5 GB port and the router 1 GB port. We read that CAT5 cable could carry 1 GB for short distances, but with nothing else to do, we found a CAT6 cable to connect the BGW320-505 and the router, and viola, problem solved.

I searched for hours trying to find a layman's explanation of how cable performance affects auto-negotiation link speed, and couldn't. What I guess is there must be some error occurrence in the auto-negotiation algorithm "fast link pulses" that tells the link hardware this speed (1 GB) just isn't going to work.