I'd strongly recommend transitioning from RMI to the REST API. With the transition of Maximo the Application Suite, making RMI calls outside of the pods isn't supported. Functionality that was previously difficult to do without RMI (such as getting the allowed value list or bulk validation of attributes without submitting) is now possible with the REST API. And you'd be able to do Native authentication, LDAP, or API key based authentication flows easily.
I understand it's not a simple flip of a switch but directionally, that's the approach organizations should be going.
I saw this post if you need to authenticate utilizing LDAP via RMI (https://www.ibm.com/mysupport/s/forumsquestion?language=en_US&id=0D50z000060eO6wCAE). I haven't done it myself but I think that will take you down the right path.