Hello! Today, I have a guide for you. As most of you people on mobile know, you can only play local multiplayer. So, here is a way play online using Hamachi.
Step 1: Log in/register to the LogMeIn preview site.
Step 2: Create a network. Name it whatever you want, and make it a mesh. Then, make it up to five members, which is free.
Step 3: Now, click “Add a Client” from your “My Networks”.
Step 4: Choose “Add mobile client”.
Step 5: Name it what you please, but make sure you know it is you.
Step 6: Check the box beside the network you want to add the client to.
Step 7: You should be redirected to “Edit Client”. Check in “PPTP”, then save it.
Step 8: Depending on your device, click on “Preview Android/iOS configuration”, then fill everything in.
Step 9: Add another client, and set it up on your friends phone.
Step 10: Once it’s all set up, all of you connect to the VPN.
Step 11: Launch Terraria, and enter your world like normal, and make sure “Multiplayer Host” is enabled.
NOTE: This will work Android to iOS, and vice-versa.
NOTE: Since Hamachi for mobile is still in beta, it has a small chance of getting a price later.
NOTE: You will not be able to communicate effectively. Only the world host can edit signs, and there is no chat.
ENJOY!
EDIT: Another note: MAKE SURE YOU USE THE PREVIEW SITE TO ADD A MOBILE CLIENT