Sorry - you mention modem. Do you mean you have a modem that you use for your internet connection or just that there’s a socket in your adapter/connection plate marked “modem”?
if the former, i.e. you actually have a modem, then the Horizon box won’t use that for its TV services but it uses its own internet connection. This points to a cable/socket problem as both your modem and Horizon box need to connect to a socket that’s internet capable, otherwise the Horizon box gets live tv and nothing else.
However, if you don’t have a modem, only the Horizon box, and you use that for wifi and connecting to the internet, then it’s more likely to be a fault with the box itself or UPC provisioning your account. Either way, you’ll need to phone them!