Can't get several routers to work

I've had Wi-Fi connection in my house for about three years now. All that time, I was using a Cisco-Linksys that, apart from the occasional reheating, didn't really give me any problems.

About a week ago, the Wi-Fi went down. I restored the router to its default configuration (aka reset). That usually did things, but this time I could not get it to work. I tried connecting my PC to the router through a cable (maybe the wireless components were broken, but the router itself was fine; don't know), but it didn't work either. I assumed the router itself was the problem, because when I connected the PC directly to the modem through cable, the internet connection was flawless.

So I borrowed a new router from an uncle, but it was the same. There was no way I could get the Wi-Fi to work. I couldn't connect through cable either. Again I restored it to its default settings, nothing. And again, the internet did work when connected directly to the modem.

I tried several things, like changing locations through the house (it's a small apartment, don't know if it matters), using three different cables, disconnecting the house phones (maybe there was interference; don't know). Finally, I concluded that maybe the two routers didn't work, since the two were used products.

Today I bought a new router (a Tenda), and it was the same. No Wi-Fi, no connection through cable. The only way I can connect to the internet is DIRECTLY to the modem, with the cable.

I've made all these tests with three different routers, two laptops, two smartphones and two tablets. The Wi-Fi doesn't exist. I don't know what else to do. I'm no expert, just done basic tests and configurations, nothing has worked.

To put it simply,

IT DOESN'T WORK
Modem---(cable)----Router----(Wi-Fi)----PC
Modem---(cable)----Router----(cable)----PC

IT WORKS
Modem---(cable)----PC