pairing a ax11000 rax200 router with ac750 mod ex2800 range extender

i am trying to pair a ac750 mod ex2800 range extender to a ax1100 rax200 router and did this using the routers setup option. It created a 2.5 ghz and a 5ghz networks.These can be viewed on devices network list, but not on the nighthawk app for the router. there it stills just shows the original 2.5 ghz and the two 5 ghz. Also when i assign devices to the created wifi networks they can not communicate with each other.I have tried connecting an Iphone to firestick to mirror movies using a refector software.