https://community.particle.io/t/most-stable-photon-fw-for-wifi-disconnections/44997

main.cpp

#include "Particle.h"

SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_THREAD(ENABLED);

SerialLogHandler usbLogHandler(LOG_LEVEL_TRACE);

#define CLOUD_TIMEOUT   60000 // how many ms are acceptable for lost cloud connection (needs to be > 45sec)
#define CLOUD_MAXRETRY      5 // how many unsuccessful connection attempts befor a System.reset
#define SECOND_MS 1000

inline void softDelay(uint32_t msDelay) {
        for (uint32_t ms = millis(); millis() - ms < msDelay; Particle.process());
}

void setup() {
        Serial.begin(115200);
        Particle.connect();
}

static uint32_t msSeenCloud = 0;  // remember when we last saw the cloud
static int retries = 0;           // how many retries to connect are left

void maintainWiFi();

void loop() {

        maintainWiFi();
}

void maintainWiFi() {
        // check cloud connection and if required reconnect
        if (Particle.connected()) {
                msSeenCloud = millis();
                retries = 0;
        }
        else if (millis() - msSeenCloud > CLOUD_TIMEOUT) {
                Particle.disconnect();
                softDelay(500);
                WiFi.disconnect();
                softDelay(1500);
                WiFi.off();
                softDelay(4000);

                if (retries++ < CLOUD_MAXRETRY) {
                        Log.info("Reconnect to Cloud (%d)", retries);

                        WiFi.off();
                        WiFi.on();
                        Particle.connect();

                        if (waitFor(Particle.connected, CLOUD_TIMEOUT)) { // only works with SYSTEM_THREAD(ENABLED)
                                retries = 0;
                        }
                        else {
                                Log.info("\tfailed - retry in %d sec", CLOUD_TIMEOUT / SECOND_MS);
                                Particle.disconnect();
                        }

                        msSeenCloud = millis(); // don't check too early
                }
                else {
                        System.reset(); // no success in reconnection - restart from scratch
                }
        }
}