The LaMetric doesn't have the relevant hardware to be able to connect to a cellular network. If you don't have a Wi-Fi connection available where you want to use the LaMetric, an alternative would be to get a cellular hotspot device (e.g. Verizon MiFi); the LaMetric can connect to the WiFi access point created by the hotspot, and the hotspot will connect to the internet over its cellular connection.
T-Mobile has a plan for ~$20/month that includes 1GB of 4G data and unlimited low-speed data (which is fine for the LaMetric), and the device will cost you ~$65 up front.
If you can write iPhone/Android apps, you could write a phone app that makes the relevant push call to your LaMetric app, which would make your LaMetric at home display the message you entered. Otherwise if you have a web server that supports PHP, you can make a small web page that you would enter your message on via either phone or computer, and it would push the message to your LaMetric app.
This has actually given me an idea to write up a simple "shim" service for LaMetric users who don't have their own web server... of course you'd have to trust my server with your access token and push URL, but you'd be able to enter text or whatnot that would be pushed to your app.
one time inexpensive unit
no monthly cost
and low bandwith but good enough for lametric!