this is sooo hard to find information on. luckily some 15 yr old kid helped me and hopefully you figured it out already too but the actual app Minecraft PE can't do external servers for some reason. But the app BlockLauncher(free) that loads up Minecraft in the background and then is able to customize it can.
Just download BlockLauncher(free) and install it. If you already have everything else already set-up in the regular Minecraft app like it sounds like you do then you're all good to go. Just open Blocklauncher and select your server from the external IP and wa-la, you're connected.
I searched for 5 days straight for a solution and then he told me, "oh ur on mobile. it won't work. use blocklauncher." which I already had installed and as soon as I did it worked flawlessly... now I just have to figure out how to implement mobs, should only take a week of searching forums I guess.