The follow problem has been occurring for 2 weeks with my Pi 3 (Pi, SD card, power supply are all brand new).
After configuring, I boot up my Pi (stretch), it connects to the internet, I am able to ping and SSH in, everything is great. I am feeding multiple ads-b sites automatically.
After a certain amount of time (varies between 3 days to 4 hours) the Pi will be unreachable via ping or SSH. The strange thing is that it keeps feeding to the ADS-B sites. So I know it's online and transmitting data.
I've tried certain scripts that attempt to detect if wlan0 is up/down and reconnect but I later learned ifup was deprecated, invalidating those attempts (I haven't been able to find a new suitable script) that doesn't force a reboot.