Step 1:
Go to the top right corner and click on edit.
Step 2:
Click on External.
Step 3:
Find an online server by searching "Minecraft PE Servers". Name your server.
Step 4:
Enter the IP address and port.
Step 5:
Now click Add Server.
Step 6: Troubleshooting
Sometimes the server doesn't connect right away, try again a couple of times.
Some servers require to type out: /register before you can play.