Hello @F. Leoni , Thank you for reaching out. Can you please let us know what kind of APP API service you are using?
If you are using an Azure Web App, it has a set number of outbound IP addresses at any given time. Any outbound connection from the App Service app, such as to a back-end database, uses one of the outbound IP addresses as the origin IP address. The IP address to use is selected randomly at runtime, so your back-end service must open its firewall to all the outbound IP addresses for your app. You can find the set of all possible outbound IP addresses your app can use, regardless of pricing tiers, by looking for the possibleOutboundIpAddresses property or in the Additional Outbound IP Addresses field in the Properties blade in the Azure portal. Please go through this documentation for additional details.
Hope this helps. Please let me know if you have any additional concerns. Thank you!