|There really isn't any point to using the WAN port for this. It just adds complexity to your setup. Plug the server into a LAN port and be done with it. This requires no configuration to allow your other LAN clients to connect to the server.|
The basic principle in all computing is: KISS which stands for "Keep It Simple Stupid"
And no, I'm not calling you stupid, that's just what the acronym stands for. What it really means with regard to your setup is, trying to utilize the WAN port for something that should be using a LAN port means having to properly configure the WAN port so as to allow traffic between LAN and WAN. If you knew how to do it, you wouldn't be here asking how and since you are, this means it's:
a) more effort than it's worth (since there is a simple, proper, way to do it
b) beyond your knowledge level and abilities and therefore again, more effort than it's worth since you either have to get explicit instructions from someone who can figure it out, or else, you have to figure it out for yourself.
I'm a networking professional. I have over 15 years of experience in computing (the last 4+ specifically in enterprise level networking) and I wouldn't bother with the mess you're attempting, I'd just plug my server into a LAN port and be done with it.