I set up  a new AC750 and it seems to have connected to the WiFi networks properly.  I see that it added two new connections which are the original 2.4 and 5G networks' names, but with the extensions _EXT added.  When I connect to those, the speed is extremely slow.  When I connect to the networks I was extending, the speed is as fast as it was without the AC750.   The extender is about 15 feet from the router and on the other side of a plaster wall.  What is wrong?  Am I supposed to connect to the _EXT networks or does it extend the range of the original networks?  I got the extender to provide faster 2.4 speed to an upstairs room and it appears to have done that on the original network, not the _EXT network.  For reference, the router and modem are downstairs and the device I need to have the benefit of the boosted/extended speed is one floor up in my home with plaster walls.  Kindly advise.

Thank you.