Adding transparent videos on top of videos in Adobe Express

Hello!

I made some animations in After Effetcs ands transported them with transparent backgrounds, so that they can be used to layer over another video for social media posts and suchs. As this is for a client, they want to work with them theirselves and the easiest way for them to do so is via Adobe Express since they are not familiair with Premiere or another video editing program. 

Now, I transported a video with transparent background succesfully and made sure it's not too big of a file size etc. But the weird thing is: when I put my transparent video over the existing "background" video in Adobe Express it shows transparent, the preview looks perfect. However, when I export the video suddenly the background of my transparent video appears again! Now I read that the MP4 format is not compatible with transparency but shouldn't this just work if I have the transparent video on top of a background? 

I hope anyone can help me with this!

Thanks in advance!

Rick