Since C2 is working as an access point, to make NTP srver work, you have to create a static route rule on C2. There is an instruction. http://www.tp-link.com/en/faq-560.html

On web interface of WA901, under System Tools, there is SNMP, right? Can you describe the problem in details?

What's the IP address of the device which you are trying to open port for? Can you have a check on the device itself? You have to ensure its IP address is 192.168.1.* which is in the same subnet as...

It is noted on TP-LINK website that TD-W8961ND_V3_140305 cannot be downgraded... There is no way to make it... Now with the firmware 140305, your modem is still running? Can you reconfigure it to...

You configured one-to-one NAT? Interface can be selected on that page? http://www.tp-link.com/en/faq-381.html

If so, Wireless Security Mode is Most Secure(WPA/WPA2-PSK)? Did you enable wireless MAC filter on the old AP? After finish the configuration, what's the IP address obtained on PC connected to WR743ND?