How long does the failover take, and will active sessions drop?

Failover behavior depends on the exact model and configuration. A router with automatic WAN-to-cellular failover will switch traffic to the cellular path when the fixed NBN link is lost, and switch back when it returns. The timing and smoothness depend on the hardware, firmware and health-check settings you enable, so it can be effectively seamless or involve a brief interruption. Active sessions may be briefly interrupted during the switch unless you configure features designed for session continuity, such as link health checks and a watchdog. Dual-SIM capability and robust remote management can help reduce disruption and improve recovery. If you want, share your site location and whether you need failover only or full-time internet; we can suggest models and configuration options that aim for minimal interruption and quick recovery.