Ethernet working well connected on laptop but NOT on router

So i have 2 identical circuits (arduino+ethernet+sensors) and 2(almost) identical codes, because the circuits do the same work, but one of them works well with the router and the other one works well when connected with computer but not on the router. I say almost identical because in the code thay have different mac addresses of course. I have tried testing the second one with setting the arduino ip address manually and also not setting the ip but rather allowing the router assign a random ip to it, and still not working (not making the connection with the web server)

I am out of ideas of what's going wrong and how to fix it.

Any suggestions? Thanks in advance

Just an educated guess: "ethernet" does not mean an original Arduino Ethernet Shield but a cheap Chinese clone. If I'm right you might have one that has the wrong resistor value soldered. These board usually connect fine to a PC but don't work on router/switches because the speed auto-selection is not working correctly.

pylon is referring to