Not possible in Sandbox, use your desktop modeler to test by adding the certificate into your settings
You are using a free version. You can find the network tab in the paid version and from there you can upload the certificate.
Another thing you can try is to go to Modeler. Under Settings > Certificates, upload the SSL certificate and use the “Run” option to run the app from cloud. That might work.
Thank you for your answers. I found a workaround for the initial problem (Mendix not being able to call my REST server):
1.) I created a dyndns-Domain for my REST-server
2.) I got a certificate for this server using Let’sEncrypt
3.) I put the dyndns-adress into each REST-call
Calling the dyndns-adress in the browser, it still says that the connection might be insecure, but given that it’s signed by a CA (Let’sEncrypt) AND that the certified name matches the URL, Mendix seems to be ok with it.
For anyone reading this wondering if Mendix accepts CA-signed pages where the certified name DOESN’T match the URL: It doesn’t, I tried.