Discovery should work with newer AirRohr firmware versions and it should also discover devices with Smogomierz firmware.
To use the discovery you need to allow incoming connections for this application.
Click 'Erase flash' to delete the SPIFFS file system (contains config and downloaded firmware updates). The actual firmware won't be deleted.
Debian/Ubuntu: add your user to the group dialout with sudo usermod -a -G dialout ${USER} (you may need to restart the desktop to apply your new group settings).
