Wireless box wifi connection

[bdh29]

I have a quick question, but first let me briefly describe my setup.

-Wireless router is on the 1st floor.

-One of the upstairs bedrooms has a PS4, so I put an extender in that room so the PS4 can connect via ethernet cable.

-The master bedroom is at the other end of the 2nd floor and has a wireless mini box, but it constantly freezes because its connected to the extender.  When I unplug the extender, it connects to the main router and works perfectly.

So my question is this: if I leave the extender running, can I manually select the main router as the signal the box connects to?

[Edg1]

The is no manual setting for wifi it should connect to the strongest signal. It is strange that it breaks up when connected to the extender. 

You could try unplugging the extender and reboot the mini and see if it stays attached to the router after you plug the extender back in.