How to share android screen with users outside LAN?
You can use BBQDroid, but it will require additional tweaking on your router.

First, you'll have to find out your IP. This can easily be done by typing "my ip" into Google, which will return your public IP.

Next, you'll need to go into your router settings and add your Android device to the router's DMZ, meaning that all incoming connections get forwarded to your device. 
Find out the model name and brand of your router, then look up instructions on how to access the DMZ.

By this point, your public IP should direct to your phone. Launch BBQDroid on your device and attempt to connect to it using your public IP from another device.

When you're finished, I'd recommend removing your Android device from your router's DMZ, as a security precaution.