If the cutomers network is set up with a DHCP server, then you leave your IP and DNS settings as automatic and let DHCP take care of things for you. If they don't have DHCP for whatever reason, you will need to set the default gateway and DNS server IP to that of their internet router. If they are using a 192.168.1.0/24 address space as you suggested, it is quite likely that this will be 192.168.1.1.