First, make sure that any residue on the lens is removed before applying the numbers. If the lens has any kind of residue on it the numbers will never adhere to the surface of the lens. Even a seemingly clean lens can have unseen residue on it so wet a paper towel with water and squeeze out all of the excess liquid. Gently clean the lens then pat it dry with a lint-free cloth.
After applying the numbers to the lens use the curved underside of a teaspoon and using gentle pressure rub the spoon several times over each number paying special attention to the edges. Finally, set the unit aside and allow at least 24 to 48 hours for the adhesive to cure. This will allow the adhesive on the numbers to create a stronger bond with the lens. Now you should be ready to set the address sign outside.