I had this issue and the best thing to do is to contact support with the number you want to port over and the PAC Code. They will be able to action that for you.
You'll have to log out of your account and go to smarty.co.uk/activate. There's a video here on how you can do it https://community.smarty.co.uk/t5/help-videos/activating-a-spare-bonus-sim-or-a-sim-bought-in-a-shop-or-other/ba-p/26238
