Have the same setup (Spectrum/Nighthawk). This works for me. No bridge or whatever needed. Should work for pretty much any Netgear router.
First, set up port forwarding like this. (Use the add custom button.)
Internal IP address should be the same as your core/server machine.
Second, not sure this is needed, but I reserved the IP address of the local core/server machine so it doesn’t get changed by dhcp.
In both cases, you can select from the list of active connections to get the local ip and mac address for your server/core.
Been working for me for a couple of days.