Thanks for your very useful comments
From this side of the pond it looks like Franklin Wireless has several devices of possible interest.
I spent a LOT of time on the phone with Sprint. Mostly noise. On the same call the price went from $43/month to $15/month. Smoke and mirrors IMHO!
Franklin describes one device as suitable for ATMs and signs. Possibly a reasonably reliable unit? I am not sure about reset/restart and the other necessary issues for an embedded design.
If I were willing to have WIFI in the chain going to the internet, some consumer devices might be useful?
And of course there is that nasty issue of DDOS. Really a network issue despite all the hype. But am I supposed to pay for data overage if my remote device becomes someone's DDOS target?
Whichever ISP 'should', on their end, block packets not from the IPs I specify
If I may ask, which devices have you looked at? And how is your design coming. Also, thanks for the Arduino tip
Lots of fun