Internet stops working when ethernet cable connected to gateway
I have the following setup and every time I connect the 3rd ethernet cable both WiFi and hardwired Internet stops working.
Setup: I have an AT&T gateway modem (WiFi through this is disabled). Plugged into the gateway is a ethernet cable to my Netgear router. Second ethernet cable going to security system.
Issue: everything works just fine until I plug into the gateway a third ethernet cable that goes to a switch in another room - from that switch I connect to a DVD player, directv box and VoIP adapter. When I plug this in, there is a visible WiFi network but it won’t let me connect and/or disconnects me and any hardwire cable stops working too.
When I remove the 3rd ethernet cable plugged in the internet will start working within a few minutes.
i can’t figure it out and not sure what I should even try
See quick diagram for reference.