try dynamic ip and see if that works. maybe your modem has already setup pppoe connection. if no, make sure username and password are correct.

do you setup this C50 as a wireless access point? if so, both bandwidth control and guest network won't work. if no, show us screenshots about bandwidth control settings. make sure you have input...

change it wan to static ip 10.0.0.1 then change its lan ip to 192.168.0.252, later disable DHCP. last connect cable from main router to lan 1 of AD7200.

yes, you can do that. about how to pair them, you can do this, pair A and B first. Then pair B or A with C. last C or A or B and D. Each time pair two devices. http://www.tp-link.com/us/faq-1618.html

hi zoran, check the dsl parameters. it they are in the normal value but you still get disconnect, then it the problem with this unit. if no,then it is the isp problem. here is the reference for you....

it is better to check the password as it won't be accept by your wifi.