connecting eth1 to ISP

I've built a proxy server in centos 7 and it connects to my internet router and works fine. My internet router connects to a fttc modem and I'd like to get rid of the router and just plug the proxy server straight into the modem. problem is the proxy server can't connect to the internet because it doesn't have my ISP username-password in it anywhere. The proxy server connects to the internet via eth1. Is it possible to somehow include my ISP details into /etc/sysconfig/network-scripts/ifcfg-eth1? or should I go about this some other way?
I've been searching google but can't find a way to do this, thanks for any help.