UN here stands for Universal I think. Then it should be a universal version for worldwide.

DHCP Server---DHCP Settings---Lease Time?? But it seems cannot set it to never...

Can you try different channels? Phone works fine with same Wi-Fi, which means router is fine...:confused:

I cann't explain but TP-LINK shoud do it. That's what it does. http://www.tp-link.com/en/article/?faqid=649

Can you ping each other successfully? HDD, printer and MAC all connected to main router can see each other?

Default gateway is 192.168.1.1? Did you get a window asking for username and password? Restart it and keep holding reset button for 15 seconds. Check status of all lights, especially System light. As...