Hi @hshifi
So, all internet traffic happens using TCP/IP. This protocol has source and destination IP addresses. The only traffic coming to your router is traffic specifically sent to you.
Some of this traffic is of course malicious and intended to look for vulnerable services. The router behind the modem is responsible for blocking all incoming traffic that is unsolicited/unwanted.
Having said this, I have seen coaxial cables cause ground loops. Having a cable isolator fixes these.
Best,
E