Local smart home on a zero data cellular fallback

If you run a local-first smart home, Home Assistant is probably the brain of it, and the whole point is to lean on the cloud as little as possible. That design has a nice side effect: when your home internet drops, most of your house keeps working. The thing a cellular backup actually saves is the stuff that does need the internet — remote access, push notifications, and any cloud-dependent integrations. Let’s be honest about what fails over to cellular, what doesn’t, and how to keep the data bill near zero without pretending there’s a magic “no data” switch.

1. What actually keeps running with no internet

Before you buy anything, get clear on what an outage even breaks. If your devices talk over Zigbee, Z-Wave, Thread/Matter-local, or ESPHome, they communicate directly with your Home Assistant hub on your LAN — the internet is not in that path. So when the WAN goes down:

• Still works: local automations, Zigbee/Z-Wave/Thread devices, ESPHome sensors, lights, locks, and any rule that runs entirely on the hub.
• Breaks: the Home Assistant Companion app from outside the house, cloud voice assistants, push notifications, and integrations that call a vendor cloud (some thermostats, video doorbells, weather, etc.).

That split is why a cellular failover is a connectivity tool, not an automation tool. You’re not buying it to keep your lights on — they were never at risk. You’re buying it so you can still check the cameras and get alerts while the cable is out.

2. Pick a way onto cellular

There is no consumer device that does “cellular with zero data.” A cellular link is a real LTE/5G connection through a modem and a SIM, and it uses data whenever traffic flows over it. The trick is to keep that traffic near zero by only using the link during an outage. A few real options, easiest to most capable:

• Phone hotspot. The cheapest backup. Tether a phone’s hotspot to your router’s secondary WAN. Fine for short outages; just watch your plan’s hotspot cap.
• A travel router with a USB modem. A small OpenWrt box like the GL.iNet Mango (GL-MT300N-V2) supports tethering and 3G/4G USB modems, and can do multi-WAN failover from a wired uplink to the cellular dongle. Modest throughput, but plenty for HA traffic.
• A dedicated cellular gateway. The GL.iNet Spitz AX (GL-X3000) is a Wi-Fi 6 router with a built-in dual-SIM 5G NR / 4G LTE modem and built-in WAN/SIM/repeater failover. For business-grade hardware, a Cradlepoint does the same job.

Drop the SIM into the cellular device, and put a cheap data SIM or a low-cost IoT/backup plan in it — you won’t use much.

3. Make it fail over automatically (and only when needed)

The goal is a link that sits idle until your primary WAN dies, then carries traffic, then steps back when the cable returns. Any dual-WAN / WAN-failover router does this:

• OpenWrt uses mwan3: give your primary WAN a metric of 0 and the cellular interface (usually wwan or ppp0) a higher metric like 10, so cellular is only used when the primary can’t route. mwan3 pings targets to decide when to switch. Test your real config — some setups detect the outage but need tuning to fail over cleanly.
• UniFi gateways, pfSense, and OPNsense all have dual-WAN failover with the cellular link set as the backup gateway.
• The GL.iNet gateways above and Cradlepoint have failover built into their own firmware.

Because the cellular path is the backup, it draws essentially no data while your main connection is healthy — that’s the closest thing to “zero data” that actually exists. Bonus: with mwan3 you can write a script in /etc/mwan3.user to fire an MQTT message on failover, so Home Assistant knows it’s running on the backup link and can react (for example, switch to a low-data mode).

4. Keep data use down on the backup link

Cellular data is metered, so treat the failover link as precious:

• Cap it. Set a data limit on the SIM/plan, or use your router’s traffic limits, so a runaway upload can’t blow the budget.
• Pause heavy uploads during failover. Disable cloud camera uploads, off-site backups, and large media syncs while you’re on cellular. If HA knows it’s on the backup link (see the MQTT trick above), an automation can flip those off automatically.
• Trim notifications. You want alerts, not chatter — high-frequency sensor pushes add up.

Done right, an outage costs you a few megabytes of remote access and alerts, not gigabytes.

5. Remote access during an outage

All the usual Home Assistant remote-access methods work over cellular, because to them it’s just another internet connection:

• Home Assistant Cloud (Nabu Casa). One toggle, works behind CGNAT — which matters, since many cellular plans put you behind carrier NAT. This is the simplest option and it funds HA development.
• Tailscale. The official add-on builds a WireGuard mesh between your devices; works through CGNAT and uses very little data when idle.
• Self-hosted WireGuard. Roll your own tunnel via the add-on if you’d rather not depend on a third party. Note: a plain port-forward to your home is often impossible on cellular because of CGNAT, so prefer Nabu Casa, Tailscale, or WireGuard with a relay.

Related products

Local-first picks from our catalog that fit this topic:

• HomeWizard P1 Meter - sensor
• Amcrest SmartHome AD-110 Video Doorbell - doorbell
• Aqara Smart Lock U100 - lock
• Aqara Smart Video Doorbell G4 - doorbell
• Athom Smart Plug V2 (ESPHome) - plug

Quick verdict

A cellular failover is a useful safety net for a local-first smart home — just for the right reasons. Your local automations already survive an internet outage on their own. The backup link is there so you keep remote access, notifications, and cloud integrations while the primary WAN is down. Use a real LTE/5G modem or travel router on a dual-WAN/failover setup so it stays idle until needed, cap the data and pause heavy uploads while it’s active, and rely on Nabu Casa, Tailscale, or WireGuard to get back in. There’s no “zero data” mode — but a well-configured failover link comes about as close as physics allows.