MariaDB - Replication - Unable to login remotely
I think this is a good clue to what is wrong:

I also noticed I am unable to login to the master from the master if I
specify its IP address rather than just localhost.

It seems as if you don't have the right host specified for the user you have created. In MariaDB and MySQL when you create a user you also need to specify a &quot;host&quot;. This can e.g. be a specific IP address to allow connections from that IP address only, or it can be a wildcard to allow connections from all hosts, or a partial IP address combined with a wildcard:
CREATE USER replication_user@'%' IDENTIFIED BY 'super_secret_password';