MQTT_CONNECTION_TIMEOUT, which doesn’t particularly help, but I have another thing to try. Since you said you’re using an older WiFi, you may have to enable an option in the underlying PubSubClient library that we use. From their readme:
Arduino WiFi Shield - if you want to send packets > 90 bytes with this shield,
enable the MQTT_MAX_TRANSFER_SIZE define in PubSubClient.h.
PubSubClient.h will be located in your Arduino’s library folder. On a Mac, it’s at:
~/Documents/Arduino/libraries/PubSubClient/src/PubSubClient.h. Open this file and uncomment out the following line (roughly line 42):
#define MQTT_MAX_TRANSFER_SIZE 80
Save the file and recompile and upload your sketch to the Arduino to pick up the change.