have to continually reset nighthawk ac 1750

You have 2 mesh extenders and a router. Some details are needed and then some pointers. 

1. firmware on the R6400?

2. Firmware on the EX6250?

3. Firmware on the ex6400?

4. are the extenders setup in mesh mode? 

Now some pointers. 

1. Extenders drop throughput by 50% for single/dual band extenders. This happens because the extender has to go router-----extender and then extender----device and can't do both at once. So it cuts the speed in half. Plus you have to take into account distance/obstructions/interference. so its already running at a reduced speed from what the router is putting out. 

2. daisy chaining (router---->extender----->extender) makes it even worse because the 2nd extender puts out 50% of what its getting from the first extender. Plus you have increased latency. 

3. 15ft and 20 ft are pretty close together. If the extenders are setup in a mesh, they can end up connected to one another or even just one daisy chaining off the other. It creates instabilities and slow downs. Its why you'll find us recommending moving to a mesh system like orbi or mk nighthawk for those needing more than 1 extender. 

4. Try setting the router with its own ssid and the extenders with the same ssid. This forces them to connect to the router and not to each other. Or power one off and see if it stays stable.