I don't understand what you mean by 'disconnection'.
When you configure 'WAN-to-LAN', you should enter
settings for both WAN and LAN. Typically for a CATV
modem, you use DHCP for its [WAN] setting; the LAN
port(s) will be at 192.168.1.1 by default, with DHCP enabled.
After you run the wizard, you plug the WAN port [eth0]
into the modem, and connect your PC to eth1, set to
DHCP client. Have you done both of those things?
If so, what is disconnected? What IP addresses are in use? Dave