That is really slick solution, though if you’re in the middle of no where (i.e. you have a contract to maintain a little league field and there’s no connectivity), the unit is going to be very likely exposed to the elements. Doesn’t look like that enclosure is IP67 graded 😉
I’d be curious to know what sort of DB losses you end up suffering when you end up picking up a proper weather-proof enclosure. Sadly it doesn’t look like there’s a readily available SMA or type-N aux port on that router, so you can’t just do the ol’ “throw it into a properly rated enclosure, drill and tap for your water-sealed fittings. Realistically though GSM is at 850mhz, so you won’t really have any major comms issues I’d imagine.
An even simpler (but cruder because you lose connectivity to OpenSprinkler– though you do gain the granular control of what you are tx’ing is to fire off information via stateless UDP packets to a 10$ VPS. If you have sporadic connectivity, there’s no losing and having to renegotiate a TCP session. Just a simple one-two
1) Use an off the shelf GSM shield that uses standard UART. It’s tuned for quad-band, so it’ll work with basically any provider on the planet.
2) Serial tx/rx to ESP8266 via UART to dispatch any pertinent infomation you need
Bobs your auntie.
 http://www.intelliconnectusa.com/connector-products/sma-waterproof 😉 Overkill much? I’ve never seen these used except in portable spectrum analyzers meant for field service, that run like 20k a unit from Rohde & Schwarz
 https://www.arduino.cc/en/Main/ArduinoGSMShield The pertinent information is : “The shield uses digital pins 2 and 3 for software serial communication with the M10. Pin 2 is connected to the M10’s TX pin and pin 3 to its RX pin.