'Remote Desktop' control between android phones over cellular

[YaSo]

I am trying to find an app that lets me use a phone (A) in one location / city and access the screen functions (screen mirroring) of another phone (B) in another location / city over cellular networks.

I have seen many apps that let you do this between local computers and Android devices (Google Desktop Remote for one) and have seen Android to Android apps that work over close WiFI or Bluetooth proximity.

But I cannot seem to find one that will let me use my phone to access / control / mirror another phone far away.

I don't want to transfer files, but to use phone (A) as if I was using phone (B) while it's in another city.

Does anyone know of something that will do this? And while I consider myself a 'techie', my boss also needs to use this so it should be something that once I set it up, should be fairly easy to use.

Thanks!