hi all, i'm not the most technical, i'm hoping someone can help me.

my setup is 

an old archer modem/router acting as a modem only. with the archer be3600 plugged in and using as a router. everything is fine, everything is all connected and showing in the tether app.

the range on the router isn't brilliant so i bought another router to act as an extender. i configured that as an extender in the menu and that is now connected to my network and shows up in the tether app. 

how the hell do i use that extender?

my iphone and other devices can't see it when searching for wifi, so i can't connect 

any help much app