The connection you configured in UaExpert is using X509 authentication, not anonymous authentication. The key and certificate used for this mode of authentication is not the same key and certificate used to secure the connection between client and server (the Application Instance Certificate).
Ignition’s OPC UA client does not support X509 authentication. The advanced configuration option you uncovered is to allow a different Application Instance Certificate to be used by the connection.
If you want to establish an anonymous connection then simply leave the username and password fields blank. But this is not what you did in UaExpert.