https://192.168.x.y:8123/ is only available of you set up SSL either with letsencrypt or with self signed certificates. Be aware that lets encrypt needs an public accessible URL to complete the acme challenge and self signed certificates cannot be created without some knowledge about SSL. But there is an step-by-step guide: Certificate Authority and self-signed certificate for SSL/TLS
The probably easiest solution is to use nabu casa. SSL comes out of the box here. Just set up your internal and external URL accordingly afterwards and your good to go.