@YARVOLK 

It is not that you are doing anything wrong, it is just what happens when you have two routers with the same network name. If you were looking for a network where you can roam and your device would move nodes seamleslly I recommend looking at our Deco Mesh devices. 

In your situation the two router can not dictate or tell the client device where to connect to, it all depends on the device itself. So like you mentioned your device will stay connected to a AP even when it is closer to another AP. You can possibly see if that client device is able to be adjusted to were it can switch based on network strength? Other then that your other option is to name both networks different and manually switch.

@Tony Thanks for your reply.
 

There are a couple of working modes, that a router can be working on. One of them is – "range extender mode".
I think I got a wrong understanding of this mode. I was expecting it to extend my current signal range, not duplicating it.
If that's how it should work, then it's fine. At least, I know now that is not what I wanted...

Thanks again.