Your plugin firstlastname doesn't work with invite only. No way to write in first name or last name.
Please take a look at this comment and see if it is helpful for you: https://vanillaforums.org/discussion/comment/233928/#Comment_233928
The plugin seems to have a Security Vulnerability and it does not look to be updated since the vulnerability has been found several years ago.
probably best to steer clear of the plugin unless you have a strong desire to have your forum compromised.
Suggest author delete plugin, since it has a reported security vulnerability.
Pragmatism is all I have to offer. Avoiding the sidelines and providing centerline pro-tips.