While helping a customer searching for travel router, I came across some travel routers that have 3G/4G support, like this one:
https://www.amazon.com/TP-Link-Wireless-Portable-Router-TL-MR3020/dp/B00634PLTW
My understanding is that it will allow 3G/4G connection, and share that connection through WiFi or the built-in Ethernet port. I have not tried it yet, but just thought to bring this up because this may be an easy solution to get OpenSprinkler connected to cellular network.