Some internet service providers flash custom firmware that locks out WAN modifications entirely. If you cannot edit the WAN profile or create a new one, you will need to contact your ISP's technical support line directly and request that they remotely switch your ZTE F689 to bridge mode from their side.
This is convenient for a basic setup, but it can become a bottleneck for advanced use. If you connect a second router (like a high-end TP-Link, Asus, or a mesh system) to an F689 in router mode, you create a "Double NAT" situation. This can severely disrupt online gaming, prevent port forwarding for web servers, and break protocols needed for services like VPNs or Roon ARC. Zte F689 Bridge Mode