R6300 v1 Cannot Connect Directly to Cable Modem Anymore

The original yellow ethernet cable that came with my R6300 router was broken for sure. When I used it to connect the internet port on R6300 to my cable modem, the internet LED on the R6300 router was off. Of course, there was no internet connection on R6300's LAN ports and WiFi.

I have another good ethernet cable. I used this one to connect the internet port on R6300 to my modem, the internet LED on the R6300 was on, but stayed in amber color, not the green color. Again, I had no internet connection through the R6300.

I used an old and functional 54G router to connect to my cable modem. I had internet on all LAN ports of this router and on 54G WiFi. Then, I connected the internet port on R6300 to one of the LAN ports on 54G router with the good ethernet cable and the internet LED on R6300 became green. I had internet on all the LAN ports and on WiFi on R6300 this way. Of course, I don't need two WiFi signals. I turned off the WiFi function on the 54G router. The 54G router basically acts as a switch between the R6300 router and the cable modem.

Very strange. I don't know what is going one with my R6300.