Ok, so I did the following to get the AC5300 connected to a Motorola SB 6180 cable modem this evening:
1. Power OFF the AC5300. May NO changed to the router before turning it off.
2. Power OFF the SB6121.
3. Connected the SB6180.
4. Connected a Mac Book Pro to the 6180. Then power ON the modem and let it sync.
5. Registered the modem with Cableone.
6. Powered OFF the 6180.
7. Reconnected the AC5300 to the 6180.
8. Reconnected the Mac via LAN wire to the AC5300.
9. Powered ON the 6180 cable modem. Came to ready.
10. Powered on the AC5300 and started a timer.
I saw the same LED behavior with the Internet LED. It came on immediately after turning on the router. Then turned off for about 2 minutes. Then came one then I saw the Power LED turn OFF then back on, then the Internet LED went off again. Saw this about 4 more times. Going passed the 6 minute mark, I let the router go to see if at any point the Internet and LAN LED would come on. After 10 minutes I went and had a quick bite to eat for dinner, about 10-15 minutes. I came up to find that the Internet LED and LAN LEDs were ON, however the internet LED was RED. Again, I had to use the Setup wizard to get it to finally connect on the 2nd scan detect attempt. After that, the Internet LED turned white and now I have Internet fully.
Log here:
https://www.dropbox.com/s/3e0gq2df418wirv/ASUS_AC5300_PowerON_with_SB6180Modemsyslog.txt?dl=0
RM, please review the log and let me know what you think. I had hopes of maybe this might resolve the what others have called problems with older model cable modems and this router. So I wanted to test and see. Seem like this didn't resolve anything. I'm wondering if maybe I do have a bad unit after all. I'll do one more REBOOT test, if I can't tonight, tomorrow.
Thank you.