Log into the 8300 and look at its WAN address. If it begins with 192.168, 10, or 172.16-31, that is a private address and your modem is not in bridge mode. If that is the case, try logging into the modem; see https://www.router-reset.com/reset-manuals/Arris/TG1672G#factory-default-settings , either to fix the Wi-Fi or to set up bridge mode.
If the modem is bridged, try running Wireshark on the Mac host and capturing a failing call. We can then see whether RTP is being sent out, whether it’s properly formatted and going to the correct IP and port, and whether it actually contains voice. If all those are true, the router is likely at fault.
Or, do you have something else you can connect the Mac to for testing, e.g. your smartphone acting as mobile hotspot, your neighbor’s Wi-Fi, a VPN (commercial, or to your workplace or school), etc?