In the PLEX Troubleshooting Remote Access they have a small section on setting up for Double-NAT, but don’t really get into how to actually do it. In my network I am using a Double-NAT setup with FIOS.
I use the FIOS AC1750 router from which I run an ethernet connection to a Buffalo WZR-HP-AG300H running OpenWRT to which I have my PLEX server and other internet devices connected.
Here’s a simple diagram of my network.