The RN-171 WiFi module has ended up stuck in WPS mode and is practically bricked - I cannot get it out of this mode at all. I suspect it has something to do with removing and plugging in the module with power connected... Lesson learned. I have switched over to an ESP8266 module which works pretty well. Only problem with WiFi is that it is power hungry. At least extending the range was pretty easy with an off the shelf range extender.

I did some initial testing with the solar panel and found that the battery was running out even on a sunny day. Measurements revealed that the unit is consuming around 160mA - the solar panel was only delivering around 50mA. No wonder. This was with no low power mode and running continuously.

I have since added low power mode and the unit only wakes up every 30 minutes during the day and every 3 hours during the night. Current consumption in low power mode (Wifi low powered as well) is between 5-10mA - still high, but nowhere close to what it was.

Next step is to clean up the software a little and do some rough outdoor testing to get an idea of battery life and whether the solar panel charging is enough. Then mounting it in the postbox and putting it to good use.