Connect the USB dongle to the back of the phone, you will see a note on the screen:
Navigate to the Menu
Navigate to DECT Settings
Navigate to Handsets
With 1. Handset 2 highlighted, press Start Reg
You will see that registration pairing has started:
On the handset, press Reg
You may need to navigate into the settings of the handset if Reg is not an option.
Settings > Registration > Register Handset > Base 1
You will see the handset register via Easy Pairing
You will now see on the desk phone that the handset is Registered, go Back
Navigate to Satellite Mode
Set Handset 2 in the Select Handset field and press Save