There is ADSL modem and Fiddler running at the computer connected to this modem.
Inside modem configurations there is an option called Bridge Mode which enables you to connect to internet like a dial-up connection.
At computer side nothing changes only modem is turned to Bridge Mode and Fiddler stops catching any traffic. When the modem is returned back to normal operation mode Fiddler works again.
Bridge mode is the configuration that disables the NAT feature on the modem and allows a router to function as a DHCP server without an IP Address conflict.