How can I export an after effects mp4 to fit an android screen

What happens if you upload this video (unlisted) to youtube?  Does it appear as your screenshot above or is it correctly filling the screen?  If you look at the file properties of the video oin your cumpouter, what are the dimensions?

For some reason, your phone thinks this is a horizontal video and not a vertical one.  I think the issue might be in Adobe Media Encoder's preset.  In AME, once you've selected the preset, click on the blue link it becomes.  In the video tab, I think you might find the video is 1920 wide.  You can alter the preset's dimensions in the same screen.

https://youtu.be/W-usRdlcOHQ?t=114

Ok so below is how I created my document in Adobe Iillustrator (1080 x 1920 RGB)

Then I saved the file as I usually do (see below)

I then dragged and dropped it in After Effects and when I'm done, I export - add to media encoder queue

Then when I'm in Media Encoder, I choose the HD Mobile Device 1080 option and click play.

I'm probably doing a couple of things wrong since I'm a newb in the video world. But shouldn't it fit a mobile screen?

Thanks

I'm in agreement with Mylenium - at this stage, only God knows what you've done. We'll need to look at details of your Render and Output, in the Render Queue. We'll also need to look at your source footage details and Composition Settings. 

Impossible to say anything without knowing exact render settings and stuff. You probably simply picked an unsuitable output preset that has built-in size constraints.

Mylenium