Sorry but I don’t understand the issue. As the plugin support said, the “Member Types” field appears by default in the “Account Details” section, and the screenshot you sent is displaying the field in that section as expected.
I also tried it with the default Twenty Nineteen theme, and the “Member type” field is still in the “Account Details” section as expected. If you want, you can see it by switching the one of the default themes (like Twenty Seventeen or Twenty Nineteen) and open your registration page. You’ll notice that the “Member Types” field is still displayed in the same section, which means that Gwangi is not responsible for the position of that field.
I hope this information will help you in your setup.