No Internet Access With New Router

I have a little problem with installing my new router. There were quite a lot of problems with the old smc, so I've bought a TP-Link router. I installed it just alright, it said congratulations, you have internet. I don't have though, not even on the directly plugged computer. It's kind of frustrating, since the old router is still working like before. So I plugged an ethernet cable from the old router's lan to the tp-link wan and with that, the new network is working just fine. The problem with the internet access occurs only, if the router is directly plugged to the modem. (The computers do find the network but the connection is kind of offline.) I don't really know what could be the problem, since I'm a little new to routers. I've read almost everything in the new router's manual and I'm pretty sure it's installed correctly. (ISP username and password copied from the old settings, mac address copied, all ip addresses pinged, done power cycle etc..) I also tried all of this on two different computers, in case eset is in some magical way blocking the router. Turned out this wasn't the problem. Well, I may have overlooked something though. I thought about resetting the modem, but I'm a bit afraid of losing the old router's internet connection as well.