How do i select either my main wireless router or my satellite

That's a more complicated question than it seems.

• Moving a device from the router to a satellite has no effect on total throughput unless the router is so overloaded that it cannot service the devices.  Any traffic through a satellite has to go through the router and internet modem (and back).  Unless a device appears to be not functioning the way it should, changing which access point it connect to isn't productive.
• Each device scans for WiFi access points, carries on a dialog with each, and chooses which one to connect (to). 
• There is one situation that can be explored.  Devices that are designed to "move", such as cell phones, tablets, laptops, often are programmed to check every few seconds for a "better connection."  In that case, when they get closer to a satellite than to the router, they may choose to move.  (My smartphone and my tablet both do.)  Devices that are not expected to move around are often not programmed this way.  They do an initial search, pick an access point, connect, and "stick."  Since the router "comes up" before the satellites, this can cause a device to "see" the router first, connect, and stick.  A useful experiment is to power off a device.  Turn it back on, and see where it connects.  With both the router and satellites broadcasting, the device may in fact choose the satellite.