R6300 v1 Cannot Connect Directly to Cable Modem Anymore

I had used a R6300 v1 router with an old cable modem, Motorola SB5100, for over 4 years without any problem.

A couple of weeks ago, the internet LED on R6300 went out. I thought it might be an ethernet cable problem and tried different cables, both straight and crossover. The light went back on, but stayed at amber color. Of course, there was not internet connection between the modem and the router.

As an emergency measure, I pulled out an old 54G router and connected it to the cable modem with a different straight ethernet cable. I got the internet back. So the cable modem was still OK and the original yellow Netgear ethernet cable was broken for sure.

Out of curiosity, I connected the WAN port on the R6300 to a LAN port on the 54G router with an ethernet cable. Surprisingly, the internet LED on R6300 became green and I had internet connection again on R6300 router, both wired and wireless. Either straight or crossover ethernet cable worked this way.

Now, I'm basically using the 54G router as a switch for R6300. While this setup works, I am wondering what could be wrong with my R6300 router. Is the original yellow Netgear ethernet cable somthing special?  Or, there is a hardware problem inside the router.