Mediacom internet issue

Weird issue while trying to set up internet at a clients house. Using mediacom internet. it recently quit working on him, so I went to check it out. The client has replaced the modem, so working with a brand new modem, swapped out the cables, check everything else. What's happening is every time I restart the computer, it comes back up with no IP address. Just 0.0.0.0 . The IP is automatically releasing for some reason. On ever computer, and even the router did it after I tried plugging it in. I try to renew the IP, and it cannot find the DHCP server. So every time, I have to unplug power on the modem, plug back in, then it lets me renew the IP. Once I do that, the internet works, both directly from the modem, and through the wireless router. But I have to go through that same process every time I reboot the computer. Unplug the modem, renew the IP address. It's very inconvenient and I can't think of anything that would cause this issue. I'm thinking it has to do with something on Mediacoms end, but Idk what it would be, and when we contact them, they just say it's our end. I need to know where to go from here. It can't be the computer, because the same thing happens on every device I connect to the modem. Any ideas?