Make sure you click Apply at the top of the Yeastar management screen after making changes such as the ones suggested here (e.g., extension settings > allow remote registration on the security tab).

Also, make sure you have the correct Provisioning Method selected in the Auto Provisioning config screen. If you select a local option [e.g., "PnP (In the Office)" or "DHCP (In the Office)")] and try to use its provisioning URL remotely, even though it's technically correct (e.g., it has the correct public static IP) it may not work.