On the Yealink handset go to Settings,

Scroll down to System Settings

Scroll down to Repeater Mode

Choose RT30

The system will now want to reboot, choose ok

Once the system is backup and you have 3 solid lights Press and hold the page button at the top until the led for the phone on the unit flashes. Top LED on this unit

Now power on the RT30 base. Once the dect, and power light are solid on the RT30 and all 3 lights are solid on the Yealink main base, the pairing is completed, It may take a good few minutes to achieve this