Connecting to the garden watering system with your phone over a private WiFi works wherever you are on the planet, even if there is no local cell phone coverage (LTE, 5G, etc).  You can even use a tablet to interact with the system; just need a device that can connect over WiFi and present the UI in a browser.

But what if you do have a cell phone signal at your garden location?  And you want to monitor/control the system remotely?

With a little bit more work we can use a cell modem hat with the Raspberry Pi to connect our watering system to the Internet via the cell modem.  Cell modems (LTE/4G) for Raspberry Pis are available for the $125 price range.  A SIM card is included for $2/month + data.  In our case, the data is minimal.  Network traffic is only generated when watering, or when the user makes some UI change.  Other than that, the network traffic is quiet.  So basically $2/month because data is negligible.

If we add a cell modem to our solution, then we can access the UI over the Internet, from anywhere.  We see the same UI as before when we connected with the local WiFi connection.

How does this work?

The same software we're running on the Raspberry Pi can also be run on another system on the Internet as a proxy for the real device.  In fact, the other system can be a hub, allowing multiple watering systems to connect to the hub, giving you one place to control multiple systems.

As I have time, I'll update this log entry with the steps to pull this off.  The trick is we're using the Merle framework to write our application.  Merle automatically provides support for this remote proxy model.  Securely, I might add, since connecting anything to the Internet requires a strong security model.  (Don't want Elves in Elveville hacking our watering system).