you may try to change wifi channel on 2.4G, if it doesn't help, firmware update is necessary

You can also consider to buy a wifi range extender or powerline starter KIT to extender WiFi range

Is there any problem when you connect the iphone to any other WiFi? you can also try to reset the modem router and reconfigure it, or check whether there is firmware update

That's not normal, did you pay attention on the lights on the extender when it stops working? And do you need to reset to factory default and reconfigure it? Or do you need to unplug it and plug it...

Do you mean the log in password? It seems cannot be changed remotely due to the protection mechanism?

You need to use the utility to do this. Go there to download utility: http://www.tp-link.com.au/download/TL-WPA4220.html#Utility Here is the instruction: http://www.tp-link.com.au/faq-398.html