When using a mac or Mobaxterm I have always had to specify -l root or whatever username I want to connect to the server with. This gets very annoying, but there is a simple fix in ~/.ssh/config you can add the following:
which will make it always use root as default. You should also probably add
ServerAliveInterval 30 ServerAliveCountMax 4
which is very handy for keeping ssh sessions from timing out. Basically this just pings the server every 30 seconds, to let your firewalls know your still alive and not close the connection.
– Preston Garrison