Im behind a NAT too
I seto n evirtual machine with the pfsense firewall using 2 NICs, then i forward every port / service for the LAN IP inside on the server
I have tested many times the multiple server setup, and always stucked in many problems... Now i`m testing one dedicated server with all stuff, i have some posts with problems like, ftp / webdav / sasl on log, etc... But i`m still trying fix it... Hard without knowledge...
I`m still learning too, but if i`m right the second DNS is useless, usualy the second DNS is for an second static valid IP