A single internet line is a single point of failure. One outage mid-meeting, mid-upload, or mid-exam and you're stuck reconnecting by hand.This guide gives your network a backup that takes over by itself. Fibre stays primary; the moment it dies, traffic moves to 5G in seconds — no manual switching, no session you have to babysit. When fibre comes back, it switches back.I built and tested this on real hardware — a repurposed fanless appliance with no video output — and validated it the only way that counts: by physically pulling the fibre cable and timing the cutover.What's inside (11 pages, PDF): Part A — Installation: make the boot USB, install OPNsense over a serial console (the headless, no-video case), first boot and web access. Part B — Failover: gateways and monitoring, the gateway group, and the one LAN rule most tutorials forget — without it, nothing works. Exact menu paths for every step. A real test method, plus a troubleshooting table for when it doesn't switch. Straight talk on what this is (link-down failover) and what it isn't (SD-WAN). Who it's for: Homelab and small-office setups that can't afford downtime. Anyone running OPNsense who wants automatic redundancy. People who want to understand why each step exists, not just where to click. Includes both the English and Portuguese editions in one download.No fluff, no 40-minute video to scrub through — just the exact steps, in order, from someone who actually ran the cable-cut test.