Model : TL-WR740N

Hardware Version : Not Clear

Firmware Version :

ISP :

Hello everyone, I'm new to this forum. Recently I can't handle my wifi modem. When I connect the modem from the IDSL to the router, the router says that I have to use Dynamic IP. When I plug the ethernet cable to my computer they still work. This sometimes happens and a solution is to turn off the router, unplug the ethernet cable, turn on the router and plug back the ethernet. But now they just won't work and forces me to enter Static IP! I don't even know how do I enter the Static IP since I can't contact the ISP. Please help asap.
P/S: The ADSL model is TP-LINK TD-8817. It uses PPPoE for internet and the internet works well with the USB (USB is 10x faster than Ethernet). Also here is how my LAN conf. screen in the ADSL looks like: http://i.imgur.com/shCA6Rp.png
I can post a screenshot of the wifi router if anyone needs. Also I think this is the wifi router's problem because ethernet works still well. Thanks.