iPhone Won't Connect to Wifi Extension

I have a signal amplifier/wifi extension(I don't know the name for it) set up and it works just fine with my laptop and such but my iphone (4s) refuses to connect to it, it will only connect to the main router set up. Is there a reason and is there a way to fix it?

 

(How would I check if it's transmitting at 5 GHz?)
The iPhone can see the extender except whenever I try to connect it just constantly tries to connect and never actually connects. I use Netgear and the extender is also by Netgear it's called the Universal WiFi Range Extender WN2000RPT. I've tried rebooting the extender multiple times and each time it didn't work, however I just tried it again and my iPhone finally connected, which doesn't make any sense because I didn't do anything different. Maybe there's a problem with the extension? I've found multiple times that it gets screwy at times—there are three icons with lights on the device one with arrows going up and down, another with something that looks like a computer with waves coming off of it, and a third with range bars; sometimes that third icon glows red or orange after staying green for a while as if it can't withstand transmitting for more than a few days. I don't understand why it keeps doing this.

But even so, even when I rebooted the extension yesterday it didn't work. So I'm rather confused about what's going on.

 

Off and on again. I don't know how to get it to stay working.

Is Insider also for Macs? How do you change to wireless N 2.4GHz? Do you do that using Insider? I'm feeling super incompetent haha. How would I change the router channel from "auto" to 11 and do I do that on my computer or on the iphone?

I tried reseting the nextwork config. and it didn't do anything it still connects to only the main router but not the extension.

Yes there is NETGEAR and then the other one is just extend.

I'm beginning to think it's a problem caused by the extension but I wouldn't know how to fix that.