First of all, of course, it is the simplest. Post your contact information in the advertising space to let people know how to contact you, and they will naturally call you if necessary.
The second is to find a trustworthy advertising company and let others help you sell this advertising space.
Third, go out to pull business, find businesses that may need these advertisements, and recommend their own advertising resources.
Outdoor advertising space is a bit deep, and there are many hidden rules in it. Learn slowly.