My switch does not work. All brightness LEDs are flashing, and I cannot turn the lights on and off.

Upgrade firmware for your HomeKit devices! We have released new firmwares for HomeKit devices to drastically improve the connection stability. Please refer to www.meross.com/support/FAQ/447.html and upgrade your HomeKit devices.

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This means that you have connected Traveler-1 from the master switch to Traveler-2 of the add-on switch. Switching the wiring positions of T1 and T2 on the add-on switch will solve this problem.