Surely it is up to your ISP to fix?
If they have provided you with a modem and a wired connection, they should have configured it for you, no?
Perhaps the problem is that the WiFi Access Point (WAP) was functioning as the DHCP server, and your new modem isn't.
Maybe you could get around this by assigning a fixed IP address, subnet mask, default gateway and DNS servers, but you should not do this without receiving an approved IP address from the network administrator.
SD