Hello, the situation is like this, i have 3 devices:
- 1 Wireless ADSL Modem/Router TD-W8961N
- 2 Wireless Routers: WR641G/642G v4 (Which i will call Router A) and a WR720N (Router B)

My ISP works with Dynamic IP.

If i connect the Router B to the modem everything works fine, the modem assings an IP to the router and the router handles the other devices that are connected to it.

The problem starts when i connect the Router A to the modem, by default the Router A LAN IP Address is 192.168.1.1 which is the same one the Modem has by default and makes Router B to connect to Router A instead. 

I changed the Router A LAN IP address to 192.168.2.1 and everything started to work fine again, Modem is giving IP address to Router B.

What i noticed was that although Router A has internet connection and every device that is "connected" through Router A's Network also has internet, they are not being given an IP by the Router A but the Modem instead, i also noticed that the Modem does not give an IP to Router A at all, it seem that Router A is forwarding every device to receive an Ip by the Modem instead but i haven't touched that at all, everything is default but the wireless settings and LAN IP. Another problem that is caused because of this is that everytime i restart the modem, Router B connects to Router A Network, so i have to Release/Renew so Router B connects to the Modem Network again.

What might be the cause that My Modem and Router A are behaving like this? and how could i solve this situation so the Modem gives only 1 IP to each Router and that each Router Manages the rest of the devices?