Yeah, there are a few ways to check for sure. The most effective is to take a device with 2 Ethernet NICs, plug it in between your modem and router, bridge the interfaces, and sniff the bridge. You can also look into ARP poisoning yourself to check whether the modem is compromised, but the likelihood of that would be slim to none (your modem doesn't have storage or enough compute to handle that kind of traffic redirection.) In all likelihood you are on an ISP that uses CGNAT that assigns a few peoples traffic to the same public facing IP address, in that case the traffic could easily be going to a neighbor that uses the same ISP.
Yeah, there are a few ways to check for sure. The most effective is to take a device with 2 Ethernet NICs, plug it in between your modem and router, bridge the interfaces, and sniff the bridge. You can also look into ARP poisoning yourself to check whether the modem is compromised, but the likelihood of that would be slim to none (your modem doesn't have storage or enough compute to handle that kind of traffic redirection.) In all likelihood you are on an ISP that uses CGNAT that assigns a few peoples traffic to the same public facing IP address, in that case the traffic could easily be going to a neighbor that uses the same ISP.