N300 wi fi plugin extender

The WN3000rp is an older N300 wireless extender. Pretty basic and lacking many features the newer devices have.

You also don't put what your router is. Potentially its got a better signal than the extender or newer standards. Also add in the fact that we don't know what your phone is and what standards it has. 

there are devices that support roaming standards but it helps if the router, extender, and device have these. 

I'll link to an article from smallnetbuilder that goes over wifi roaming a bit and some conclusions at the end. 

the easiest fix for you? (not necessarily the cheapest) would be an orbi setup or potentially a mesh extender (if your router is decent enough).

There's also a few apps available that you can program that'll autoswitch your phone based upon signal strength but its a little more complex to setup. 

https://www.smallnetbuilder.com/basics/wireless-basics/33180-how-to-fix-wi-fi-roaming