BigClown offers a lot interesting modules which you can use, but I missed WiFi connectivity - you can order module for IoT network like LoRa and Sigfox.
On the other side, this missing interface makes sense. WiFi connectivity is not low-power concept.
I accepted the higher power consumption and I designed WiFi module with popular ESP8266 IC. You can use three types of module - ESP07, ESP08 and ESP12E.
Whole article is available on https://time4ee.com/articles.php?article_id=113