from mysql server IP 10.8.12.50:

GRANT ALL PRIVILEGES ON my_database.* to 'my_user'@'%';
FLUSH PRIVILEGES;

from mariadb client IP 10.8.12.50:

mysql -u my_user -h 10.8.12.50 -p
password:
Access denied for user 'my_user'@'10.8.12.51' (using password: YES)