In many database hosting services, for security purposes, you will need to whitelist the AppSheet servers - i.e. notify the hosting service that you intend to connect from AppSheet services and supply the server IP addresses you might be connecting from.
Where within the hosting provider you need to add this white-listed server IP’s may vary. You’ll need to check the hosting providers support for those details
For the list of AppSheet server IP addresses, refer to the article below: