You are in for quite a bit of work on this one!
First let's discuss the hardware of doing this using your existing equipment.
First, all your LTE connections will need a static IP address. Compulsory.
No one (especially a public service) should use DES or 3DES or MD5 anymore. Make sure you use AES128 as a minimum. Unfortunately for Meraki we can only use SHA-1, so select that option.
You'll need to allocate each one of these devices a unique subnet, and that is your local subnet. The "peer" subnet is the subnet behind the MX that you want to talk to.
Local and Peer Id's should not be needed.
Untick the PFS option.
You can make the life times 8 hours.
And generate a random key for each site.
On the Meraki side you should have an entry for each one of your devices (each device is a separate VPN).
Now let's talk about the easy (and far more reliable) way.
Get rid of the Four-Faith boxes. They are going to make your life hard. Instead, get a Meraki MX67C.
This box can be powered from a 12VDC supply. Note that there is not an "official" Meraki guide on this, but this specific model (and the MX64) use a power supply with a 12VDC output. You'll need to arrange someone to wire up a 12VDC power line with the same plug on it (but you are going to need someone to plumb in a 12VDC line anyway).
Consequently, these units are also popular in marine applications.
If your "sites" are going to have mains power then I guess this is all a non-issue.
With the Fourth-Faith boxes you are going to have to manually configure every single one of them. And when they die, you are going to have to re-configure them.
With the MX67C you will create a template once.
And then you won't have to do anything more complicated than assign and plug them in. The saving in man-hours will be huge.
Plus you gain the capability to monitor all of the remote sites, including their 4G signal. You won't be sending people out to figure out what is wrong when a unit has gone down. You'll be able to do this remotely. The ongoing man hours required to manage the solution will be much less.