No Internet via ethernet cable connected to CM500

Hi,

I have a Cable modem cm500 and a wifi router R6080 which i working fine for the past one year. I haven't connected the ethernet cable directly to the modem after the initial setup. But recently i wanted to test the direct connection via the ethernet cable directly to the modem, which is not working. I tried with 2 separate windows laptops with different drivers, in both cases it shows unidentified network for ethernet. When i connect to wifi again it is working, ie connection between router and modem is working fine. The same laptops works perfectly when connected to the office network via ethernet. Tried reinstalling ethernet controller drivers but to no avail. what may be the reasons not connecting directly to modem? i want to avoid factory reset, but will taht even work?

thanks for any help

Roy