Access to internal web server using external domain name

I am new to Sophos UTM, setting up my home network to familiarize and evaluate the product.

I am having one issue that is really bothering me, that I cant seem to fix.

Here is the layout in its simplest form.

    [no-ip.com dyndns domain]------ [Modem]-----[Sophos Box] ----|------------[Media Server]  webserver on LANIP:25500

         media.hostname.com                WANIP                                      |------------[Workstation]

            or WANIP: 25500                                       

What i am unable to do for the life of me is access a webpage am hosting on port 25500 on my internal LAN using the external public ip address or dyndns domain name.

I am however, capable of accessing it from outside my LAN. If i use my phone on LTE data and access WANIP:25500 or media.hostname.com it works great.

Therefore i know my firewall and NAT rules are good.

What am I missing?