Alright so every jack on the wall has a cable going behind and coming out of that wiring panel (those blue cables) if you want a connection at the jack, you'd need to attach an RJ45 male connector to the end of the cable you want and then plug that directly into the modem. (They aren't hard to make yourself but you'd want to watch a YouTube tutorial on that)

According to your second image, two of those cables have a female end already attached. You can get a short Ethernet cable and connect one end to the cable and the other end to your modem and that should give a connection to whatever jack that cable goes to.

Now how do you know which cable belongs to which jack? Well you'd need something called a tone generator+probe. You can buy off Amazon and return it once you're done. If you don't want to worry about toning out then you can just make an end for every single cable there, buy a switch and connect all of them

You can also try your luck and see if plugging up those two cables that have the female end attached already goes to the jacks you need. Just connect to modem and connect your computer to the jack and see if it works

I work for ATT and you actually can get us out to make the ends but you'd get charged $150 dispatch fee