Port Forwarding

I own the cox Panoramic provided modem for Gigabit.  Modem: DTC4141

I contacted Cox Complete Care's technical support, and we went through some things and he couldn't help me fix my issue.

My modem will let me assign a static IP, and port forward to other devices. Such as, my cell phone, or my laptop connected via WiFi. Even my other router (a Netgear R7000) connected to my modem through an ethernet cable.

My desktop PC however, refuses to accept port forwarding or a static IP (I can assign a static IP through the adapter itself. It works there.)

If my desktop PC is connected to WiFi with a different IP, it accepts port forwards as well as a static IP. If it is connected (directly to the modem) it does not. Connecting to the Netgear is not an option due to range.