In the home screen, there should be a gear icon in the top right. Clicking here should open the settings for the app.
There are 2 settings that are relevant here. For local development, open the advanced settings (the "Advanced" tab) and change it to wherever the Flask server is running. Locally, it should be something like 10.0.02.2 for the hostname and 5000 for the port. Ensure that it is using HTTP rather than HTTPS.
Make sure the phone number of the CRADLE SMS Relay app is specified in the settings of CRADLE Mobile.