Apple released macOS 10.15 Catalina on October 7, 2019. This new release has stricter security requirements that require explicit permission to allow Splashtop applications to have control and work properly. Same thing for Apple's operating system up to Monterey macOS 12.
Video of setup
See step by step below
After you download and then the Splashtop_Streamer_INSTALLER.dmg, you will need to add each permission below under the Security & Privacy in System Preferences.
Our guided window helps open each section. Click Allow and follow guide.
Click the icon and enter the Mac password to unlock on the bottom left
Then under Accessibility add the Splashtop Streamer app by clicking the + sign or checking box next to the Splashtop Streamer, if already there.
You will do the same process for the other two permissions: Screen Recording and Full Disk Access.
After that you will get the success window with 3 green check marks. Your technician should be able to have full control of your Mac.