You use NAT to forward from your external IP to your internal IP - this runs into problems, when NOT accessed from the outside. Consider this packet flow:
- You (192.168.2.1x) -> 12.34.56.789
- NAT changes this to 192.168.2.1x -> 192.168.2.1x
- Requested service doesn't know about the 12.34.56.789 intermediate
- So requested service answers 192.168.2.1x -> 192.168.2.1x
- This is not an expected answer to 192.168.2.1x -> 12.34.56.789
- So it is silently discarded
Since this is a very common scenario, most SOHO routers simply do not perform the NAT for packets coming from the inside, requesting the default service instead (e.g. the internet service page)
More professional routers will allow this configuration with the unavoidable caveats, as showing the wrong connecting IP address. This is known as "full NAT", consisting of "destination NAT" or SNAT (what is called "port forwarding" in a SOHO router) and "source NAT" (which on a SOHO router is used only from the inside to the outside)