CodeIgniter：数据库连接不稳定

I have two hosts

• web host ( A server )
• database server ( B server )

Connection between them is not stable. I don't believe it is connectivity via servers problem, because I can reach that database and website anytime I want from my local device. What could be wrong?

Specification:

• MySQL 5.5.52 ( B server )
• CodeIgniter 3.1.0 ( A server )

What was done:

• Opened port for MySQL in B server;
• Granted full privileges for user to connect from A server IP to database;
• Edited my.cnf file - added B Server's external IP (in config value: bind-address)

Error Msg;

A PHP Error was encountered Severity: Warning Message: mysqli::real_connect(): (HY000/2003): Can't connect to MySQL server on 'IP' (4)