If true Bridge Mode isn't available, use IP Passthrough (DMZ). This isn't perfect (it still involves Double NAT), but it works for most users.
Call your ISP. Use this specific phrasing to get the fastest result: Zte F689 Bridge Mode
"I have purchased a high-end third-party router. I need you to switch my ZTE F689 to Bridge Mode so my router can handle the PPPoE authentication. I have my PPPoE credentials ready." If true Bridge Mode isn't available, use IP
Most fiber ISPs (like PLDT, Globe, or local municipal providers) can push a configuration update to your modem remotely to enable Bridge Mode. "I have purchased a high-end third-party router
You NEED ZTE F689 Bridge Mode if:
You SHOULD NOT use Bridge Mode if: