Two router setup with public ip to allow port forwarding
So you normally have a simple modem/router that accepts a dsl-cable.
Now, I have a project where I need to put an application online.
For this, I have bought a TP-Link TL-R407t+ and I have a Technicolor modem/router with wifi.
The setup now goes like this: phone outlet --> Technicolor --> TP-Link.
This gives me wifi from the Technicolor and the internet through the tp-link works through an Ethernet too.
The TP-Link, I have set up for port forwarding. Yet, what remains is that the TP-Link has a private ip. I am lost in figuring out how I get it to use the public IP, so that once I write xx.xx.xx.xx:8080 or whatever into a browser, my app shows up.
So my question is, how do I setup my routers, or add stuff, to allow the port forwarding to work.