Network troubleshootin’

Hi. Im using a dell sonicpoint ace router. For some reason, i get an internet connection when I plug my ethernet cord into the router, but not when i plug the cord directly from the modem to the computer. If i plug the cord into the router i get a wifi connection. If i plug another cord from the router to the computer i also get an internet connection, but i get nothing when it goes from the modem to the computer. I find that strange. I didnt set up this connection, im sort of examining an IT guy’s setup. Anyone know whats up?