This will only help if the lights are directly attached (wired) to the Pi's. They aren't of course since they are WiFi connected.
So there is no route between your Pi (or anything else for that matter) if your WiFi goes down.
The only thing that will help is a 3G dongle attached to your router.
Here is your local network:
If the WiFi fails:
No connection to the bulb from anywhere - doesn't matter how many Pi's and dongles you have.
But if the dongle is attached to the router:
When the WiFi goes down, the router is still accessible and you can reset the WiFi remotely as long as you have something set up that lets you access the router - e.g. a VPN - or SSH into the router.
Even better if you have at least one Pi wired to the router and have a VPN set up either on the router or on the Pi - alternatively, set up a NAT route in the router that exposes SSH on the Pi to the Internet (use a high, random port for the external connection so you don't get bombarded with SSH hacking attempts from bots):
As you can see, you don't really need a 3G router at all unless your broadband isn't stable.