@lroady 

If your extender is named the same as your router so you still only have one network name, the drop you are experiencing may be your device jumping back and forth from router to extender. You would want the extender to have a different network name then your main router. This will avoid doing that which will interrupt connection. Make sure to forget the other network so it stays on the current one.

@Tony 

No, that's not it. The extender network has a different name (name_EXT)

@lroady 

The extender will connect to your router like a typical client device, then extend it's own network so for your router to drop maybe something else.

Scan all available nearby networks and see if the channel the router is using is also used by other nearby devices. If so, change the wireless channel on the router, then default the extender and set it up again.

@Tony How to scan all neibour network please.

@TuanAnh219 

Use a Wi-Fi analyzer on your smartphone, go into the app store, there should be some free ones to use.

@hossners 

Yes, in your case your router is using channel 1, so is the extender once it connects to the router.

@Tony - This solution does not work.  I manually set the router wifi channel to 2, 7, and 11.  The same things happens -- the main wifi from the router drops out.  There is a serious problem or defect with your RE.  I purchased another brand solution and it worked first try out of the box.