So, I've been playing with my home internet.
I have 3 IP Addresses (at the moment) and I was having an issue with Port Forwarding with 2 out of 3 IP addresses.
I am going to do something different and point one External IP to one Server and another External IP to another server..
What I was experiencing was that one of my IP addresses was going to the UniFi OS and the other was working perfectly fine!
What was I doing wrong and how did I fix it?
My WAN Port has, 1 IP for Authentication and Connection to my ISP which I am unable to set any rDNS on that IP Address, 1 IP address with a PTR Record and 1 IP address without a PTR (Currently Pending).
Imagine that my WAN IP 87.74.X.X wasn't there... So I was running both my additional IP's on my WAN port and Inside the [Security] -> [Port Forwarding] and routing 84.9.X.1 to one server and 84.9.X.2 to another server.
I was getting UniFi OS on 84.9.X.1 but the other server 84.9.X.2 was working perfectly fine..
Getting frustrated, I found not really anything helpful online to solve my issue so whilst I was playing and seeing, it wasn't until I added my WAN IP to the Additional IP Address which I could add into the port forwarding did everything click into place.
So, having an issue like me... Shove EVERY IP you have into the Additional IP Addresses on the WAN IP and Port forward your WAN IP as well.
I didn't want to use my 87.74.X.X as a Public Facing IP because I'm unable to set a PTR record on it, but never thought about adding it as an additional IP because it was going to just be my WAN IP.
I hope this helps someone else.