Well, I'm assuming you did something like:
## Set configuration for external MySQL access
config setprop mysqld LocalNetworkingOnly no
## Apply configuration changes
signal-event post-upgrade; signal-event reboot
[System reboots]
The above doesn't give you a login.
You should now create a user with access to the database(s).
Here is an example for creating a user MyUser, pw MyPW with 'administrator' rights:
## In the command below, \ escapes a linebreak.
## Either include them, or place the entire command on one line
mysql -e "\
create database MyDB; \
GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,ALTER \
ON *.* \
TO 'MyUser'@'%' \
IDENTIFIED BY 'MyPW'; \
FLUSH PRIVILEGES;"
If this still doesn't help, you should probably check the location of the local system (is it in the local network). Although that would give you a 'Cannot connect to server' error.
If this still doesn't do the trick, please specify the exact actions you have taken to get there, so people here can see where you might have gone wrong.
Hth