Update:
Finally I got myself a new phone and my choice has fallen on a BQ Aquaris equipped with 4 GB RAM (
https://www.bq.com/en/aquaris-x).
And now, what should I say: Unity runs totally smooth, I can switch between games (which was not possible on my 1GB Motorolla Moto G2) and even big maps with some thousands of territories are playable in real time.
So probably the most important requirement seems to be enough RAM. I dont know how much you need but 1 GB is definitely not enough.
Greetings