|...but when I go outdoors away from my router my phone is looses contact with CCTV although ...|
Chances are what's happening is you're too far away from the router and it's wireless network (WLAN) and your phone loses the connection to your WLAN because the signal is too weak due to distance and interference. Lose connection to the WLAN and you certainly lose your connection to the CCTV server.
There is a limit to how far away you can be from your router and it's WLAN before the signal drops.
This has nothing to do with putting your CCTV server in a DMZ.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.