TL-WR840N is not a PoE capable device. The other option for you is to use two PoE switches between these two routers.

Since you already have AX6000, I would go for OneMesh once it's available to this device.

This feature is not implemented in TP-Link's home router devices. You can either use a dedicated firewall before you router, different router brand that has that feature or just change the service...

To set up port interval fill the external port filed (9000-9999), leave the Internal port field empty and save.

You can't and you shouldn't upload a FW from one region to another region and vise versa. Here's the FW that will fit your device:...

Check this reference to distinguish between these two - https://www.comparitech.com/net-admin/port-forwarding-port-triggering-differences/