Download the OpenWRT bin at Firmware OpenWrt Install URL (I renamed it to a shorter name, not sure if needed) and use the firmare update form of the TP-Link webinterface.
Since I already have a setup with a Fritz.box I wanted the Fritz.box to provide the network connection and let the TP-Link run as a server in that network. The TP-Link (WAN port) is connected to the Fritz.box (LAN port) via network cable. To access the SSH server on the TP-Link I need to allow SSH access through the firewall.
Webinterface - Network > Firewall > Traffic Rules > Open ports on router > Add
- Name: Allow-SSH-WAN