Problem with online

My friend and I want to play together super mario maker But one of us tries to enter the room of the other and it does the error “Unable to connect to the host.Verify that the connection settings are correct.If you still cannot connect, contact the room host and verify that the host is properly configured with the external port forwarded.”
How do we solve it?

In order for us to provide better support, we need to see the log generated by Citra. This guide will walk you through how you can obtain the log file: How to Upload the Log File.

As the error message says, the host (the person who created the room) needs to have its external port (the port used to create the room) opened on its router/modem i.e port forward.