Apologies for the delayed response.
Based on the issue description, I understand that you are having trouble connecting your front-end application to your back-end application hosted on Azure Web App.
To better assist you on this, are you using any authentication or authorization mechanism for access the WebApp? Are these two WebApps hosted on a VNET?
--You may always leverage App Service diagnostics from Azure Portal> Navigate to your App Service app in the Azure Portal. (screenshot below)
In the left navigation, click on
Diagnose and solve problems - Review and run – Network troubleshooter and ““Configuration and Management” (IP address configuration)
--To narrow down the issue, you may checkout the step-step instructions outlined by one of colleague on their own blog section: app service network troubleshooting
Kindly let us know how it goes and provide more info about your application, I'll follow-up with you further.