Although this guide is for the newer Grandstream interface, the settings generally match the older one as well.
Start by clicking on "Accounts -> Account 1 -> General Settings"
Ensure that you replace realm.nurango.io with your actual Account Realm shown under your device settings in the dashboard, or in your account settings.
To enable TLS Encryption, goto "Accounts -> Account 1 -> SIP Settings -> Basic Settings".
To enable SRTP Encryption click on "Audio Settings" and under SRTP Mode choose "Enabled and Enforced".
Ensure you have enabled SRTP in the device settings through the dashboard. You can also prioritize the G.722 codec if you want to force HD audio.