When you installed the new equipment, did you reboot your devices or update the network info? Perhaps you need to call customer service and have them provision your account to the new modem?
What is the make and model of this modem/router? And in what city, state, zip, are you located? Our community could check to see if there are noted issues with this combo unit.
First, call Customer Care and have them activate the modem to your account. I am reading that this was a self-install, so you might need to call to get the modem to hold the connection.