How to Keep the Office Online: Dual WAN and LTE Backup
Practice4 minInnovialabs editorial team
In short
Connect a second ISP over a different medium (for example fibre plus LTE) to a Dual WAN router, configure automatic failover based on reachability checks and test it regularly.
How it works in brief
Backup internet is a second line the router switches to automatically when the primary stops working. You need three things:
- a second ISP, ideally using a different connection technology;
- a router with Dual WAN (or Multi-WAN) support — two or more external ports;
- a proper reachability check that tells the router the primary line is down.
Without regular switchover tests, the backup often turns out to be broken exactly when it is needed.
Failover or load balancing
| Mode | How it works | When it fits |
|---|---|---|
| Failover | All traffic uses the primary line; the backup kicks in only on failure | Backup is expensive or data-capped (LTE) |
| Load balancing | Traffic is spread across lines at the same time | Both lines are comparable and unlimited |
| Combined | Balancing normally, everything moves to the live line on failure | Two full wired lines |
An important detail of balancing: a single connection usually goes through one line. The speed of one download does not add up; total capacity for many users grows. Some services (online banking, sites that tie a session to an IP) may misbehave if connections leave through different external addresses — set rules so their traffic uses one line.
Choosing the second ISP
The main principle is independent lines. If both ISPs enter the building through the same duct or depend on the same node, one incident can take out both.
What to look at:
- A different medium: fibre plus LTE/5G, fibre plus fixed wireless, fibre plus copper from another operator.
- Different entry points into the building, if possible.
- For LTE: signal strength exactly where the modem will sit, an external antenna option, data caps on the plan.
- Contract terms — the second line should not become a commitment that is hard to cancel.
An LTE backup is handy because it does not depend on cables in the building. But it is usually not the place for backups and updates — the data allowance can run out in an hour.
Router setup: what to enable
Menu names differ by vendor, but the logic is the same:
- Connect both lines to WAN ports (LTE through a USB modem or a separate LTE router in bridge mode).
- Set priority: primary first, backup second.
- Configure the reachability check. Do not rely on physical link status alone: the cable can be connected while the internet is gone. Have the router regularly check external addresses (ping or a DNS query) — preferably several independent ones.
- Set thresholds: how many failed checks in a row count as an outage and how many successful ones before returning to the primary. This prevents "flapping" — constant switching back and forth.
- Limit traffic on the backup: turn off background jobs on it if it is data-capped.
- Enable switchover alerts — by email, messenger or your monitoring system.
Remember IP-bound services: the external address changes on switchover. If access to something is allowed only from the office IP, whitelist the backup address too.
How to test the switchover
Test on a schedule, not just after setup:
- unplug the primary ISP cable and time how long it takes to get back online;
- check key services: email, telephony, CRM, cloud folders;
- plug the cable back and confirm the router returned to the primary;
- check that the alert arrived;
- for LTE, confirm monthly that the SIM is active and has credit.
Run tests outside working hours, but with someone who can quickly roll everything back.
Common mistakes
- Two ISPs on the same line or in the same cable duct.
- Checking only the physical link instead of real internet reachability.
- A SIM in the backup modem blocked for inactivity.
- No UPS for the router and modem — during a power cut the backup will not help.