WiFi keeps going down

I have been a Comcast customer for over 15 years and have never been more frustrated than now. We had an issue with payments on our account...once that was squared away the internet was supposed to be turned back on but never came back on. Yesterday, I swapped out the modem and internet seemed to be working great. Today, however, we keep losing our signal and the wifi keeps going down and then back up and will go down again. The modem has been restarted countless times and the connections couldn't be more secure but nothing helps. Please advise...thanks!