A solid red light indicates no internet connection. You can troubleshoot the following:

1. Check the main Wi-Fi's internet connection.

2. Restart your router, and ensure you do not block some specific ports, ie, TCP:443, UDP:123.

3. Make sure your Meross Smart Plug is not too far from the main Wi-Fi.

4. Perform a factory reset of the Meross smart device and try to add it again.