I recently contacted FM support about not being able to access hosted files on my iPhone 4s using 4G. It works fine on WiFi, just not over 4G. And yes, I do have different hosts set up for WiFi (LAN IP) and 4G (public IP), ports were opened/ forwarded.... Support finally told me to try a port scan from outside my LAN to see if the ports were configured correctly. I did this by turning off WiFi on my iPhone and my MacBook, then using my iPhone Personnal Hotspot over USB. The port scan of my host computer showed- "Open TCP port: 5003 fmp-internal". I called support back to tell them that the host computer WAS showing up over 4G. The support guy asked if he could try the scan...I gave him the IP address and he said it looked as if the ports were indeed set up as they should be. He then told me that he could see my hosted files just fine! I hadn't tried again after my port scan, but yup...there they were.
I tried a few more things and it turns out that with WiFi on, my hosted files don't show up over 4G...turn off WiFi and they show up just fine. I tried this 5 times to be sure. Once again, I do have different host configurations for my WiFi and 4G access. Hope this helps if anyone else is having this issue. By the way, I am still using iOS 5.1.1.