Full disclosure I work at ngrok. You don't have to pay for a static domain anymore: https://ngrok.com/blog-post/free-static-domains-ngrok-users

If you don't want your endpoint on the public internet, use an internal endpoint: https://ngrok.com/docs/network-edge/internal-endpoints/#:~:text=Introduction%E2%80%8B&text=Internal%20Endpoints%20cannot%20be%20accessed,active%20endpoint%20for%20billing%20purposes.