In case the custom theme icons are missing, you have to follow the troubleshooting steps below:

Step 1 - Update the icons

The first step is to update the icons. Go to Elementor > Custom Icons > theme-icons > hit "Update"

Step 2 - Mixed content error

Does your installation have an SSL certificate? If yes, the missing icons may be due to a mixed content error. You can check this solution Vamtam Elementor Themes: Mixed content SSL warnings