The WiFi signal MUST be from the modem registered on your Spectrum TV + internet account. No viewing ESPN over your employer's network, nor at a friend's house.
Are you trying to access the stand alone ESPN app or watch ESPN through the Spectrum TV app?
If it's the Spectrum TV app, you must be on your home network, but if it's the ESPN app you should be good with your existing Spectrum user name and password.
What error do you get? Can you get to the screen that asks for your Spectrum login credentials? We need some details.