If I undesrtood, you have the wan port of the router conected to the gateway lan port? then...

You need to port foward, from the gateway to the router, then from the router to the device (You should set static private IP address to the device in your lan), also if you don't have static public IP address you need to set dynamic dns like no-ip.

All this in case you have DHCP sv working both on gateway and router in a sublan.

If you replaced netgear gateway with the tp link router, just foward the port to the device in the webUI (You should set static private IP address to the device in your lan), and like above if you have dynamic public IP address

feedback

saludos