Home Network :: No Internet Through WRT54GL Router?

Recently we ordered a new Modem from Centurylink to replace an old one we have. (which ironically the old one's wireless is way better) Both modems work perfectly fine, and I've tried the same things with both to figure this out. I ran an ethernet to my gaming room, which is about 150 feet. When I connect my computer directly into the modem, my computer works fine. Initially, the PS3 worked, but when I switched back to the computer with the single Ethernet cable, I was getting an IP error. Every since then, the PS3 will not connect through Ethernet. (Wireless works, but its so far away the signal is weak)

So, I dug up my router, Linksys WRT54GL, plugged it into the ethernet that came from the Modem, (did all the steps in order multiple times), and no internet through wifi or ethernet on it. The lights and everything say there is ethernet and the WIFI/Ethernet are working, but there is no internet. All the wires are confirmed fine. I've tested them on other devices plugged directly into the modem. I tried the PS3 at the modem with 3 Ethernet cords as a test, It says there is an IP Address timeout error. Regardless of what I do, the PS3 won't connect via Ethernet. And without internet working on the Linksys router, Im getting about 15-25% signal from the Modem. (which is also wireless, if that has anything to do with the issue)

I need the router to do at the very least, WIFI. I really want Ethernet though so I don't lose much speed. (We are maxed out here at 1.5mb connection from Centurylink)

When it comes to computers, I'm advanced with them. When it comes to networking, I'm okay. I understand basic things, like accessing the router or modem through the browser. I can do the whole setup for WIFI. But when it comes to how the IP address, Static IP, etc., goes, im confused by them. So forgive me if I sound ignorant. I've looked for over a week on sites, and nothing is working. The router may be broken, im not sure. I bought it a few years ago and then didn't need it till recently. So its been sitting in a box.

All I know is, I have an ethernet from the modem to this room, the PS3 refuses to get an IP, and if I do it too much the PC gets an IP conflict. If I plug the Ethernet into the internet part of the router and do wireless or Ethernet, there is no internet coming out.