This right here. Since you can’t really configure the ISP router (1), DMZ is the way to go. The DMZ feature on home routers usually allows you to select a specific device or devices to add to the DMZ so make sure to reserve/set a static IP on your router (2) and configure the DMZ on your ISP router (1) to include only that specific IP. DMZ essentially forwards all ports to that device.
This right here. Since you can’t really configure the ISP router (1), DMZ is the way to go. The DMZ feature on home routers usually allows you to select a specific device or devices to add to the DMZ so make sure to reserve/set a static IP on your router (2) and configure the DMZ on your ISP router (1) to include only that specific IP. DMZ essentially forwards all ports to that device.