Sometimes your clients asking to you access remote mysql database from their local system, should give correct permission with specify host name, user name, and password.

Assuming that, MySQL server configure with cPanel/WHM so give grant permission carefully.

Just follow Steps to follow:

Search for "Remote MySQL" icon and click on,

By adding IP Address on "Remote Database Access Hosts" allowed external web server.

Login into your server with root access,

#mysql

mysql> show databases;

mysql> use mysql;
Database changed

mysql> show tables;

Displays mysql user list in descending order,
 
mysql>  desc user;

Allow remote permission to that user,
 
mysql>  select Host,User,Password from user where User like '%linux_user%';

Add Access Host as % (you can access database from where ever)