i think you should submit a ticket to TP-Link support so they will take it seriously.

configure the wan to static and put ip address like 10.0.0.1 to it so router won't send DHCP to wan side. pcs are connected to C2 or main router? wired or wireless? when they those internet, can they...

how many wireless clients in 2.4G? what kind of devices? does the wireless 2.4g signal disappear? try channel width to 20mhz and different channels.

try a different channel width and channels? 40Mhz ?

disable DHCP server on AD7200 then connect your main router to AD7200 in the lan port not wan port so AD7200 can work as access point. http://www.tp-link.com/us/faq-417.html

which utility you have installed on the computer? can you show us some screenshots?