Is this a leased router from Spectrum? Were you previously able to log into the Sagemcom router and now can't, or is this a new router and you have never been able to log in?
I have seen reports that Spectrum has locked out the user interface of the Sagemcom routers, especially for customers on the gig tier.
Have you tried direct connecting via ethernet to the router just in case the wifi is causing an issue? If that doesn't work, you will probably have to call Spectrum (if they locked out the user interface) or do a factory reset and reconfigure everything.
Two other helpful pieces of information, please:
What IP address is your computer being assigned by the router?
If other devices are connected to the internet, does your computer see any of them?