Maybe you dived a little too deep!
I’m assuming that you have the following setup:
The internet connection is to the modem, which in turn is connected to the AirPort Extreme. The Mac Pro and Mac mini are then connected to the AirPort Extreme using WiFi.
You only need to worry about the setup inside your local network, so in AirPort Utility you DON’T want to change the setting Internet > Connect Using, this should be setup as per instructions from your ISP.
However, on the AirPort Utility > Network > DHCP Reservations you do need to make some changes, because NET Render Server must be setup on a computer with a Static IP address. To reserve a static IP address for a particular computer you can simply use AirPort Utility to create a DHCP Reservation. If you use “Reserve Address by MAC Address” you can have separate IP Addresses for Wireless and Ethernet (recommended). Each Network adapter has a unique MAC address, which in OS X you can find in System Preferences > Internet & Wireless > Network > Advanced (LMB Click the button!) > Hardware.
You will probably need to shut everything down and restart everything to get it all working.
I suspect that it would make more sense to have the NET Render Server on the Mac mini.
Having one (or both) computers using ethernet rather than WiFi would be better from a network bandwidth and connection reliability perspective.