External screen not displaying recovery assistant from Macbook

You can try putting the laptop into Clamshell Mode and hope video shows up on the external display. You need to connect the charger and use a wire USB keyboard & mouse. Power on the laptop and hold down Command + R on the external keyboard immediately after hearing the startup chime. Also close the lid of the Display at the same time. Sometimes it can be tricky getting video to the external display at this stage. You may find things easier if you connect the laptop directly to the router using an Ethernet cable and appropriate adapter if necessary so it will see the network without requiring you to select a WiFi connection.