This worked first time on my test site, which is not to say that it's easy, but I'm pleased to report that after I'd read the docs in the context of the knowledge I have about how RFGs are stored the import was successul.
Firstly, this is the main documentation I was referring to, which is specific to using WP All Import Pro with the Types beta add-on:
https://toolset.com/documentation/user-guides/export-import/how-to-import-content-into-wordpress-using-csv/how-to-import-data-from-a-csv-file-using-the-wp-all-import-toolset-types-add-on-plugin/post-relationships/
My setup:
I made a custom post type "Businesses" and added an RFG "Offices", which has custom fields for City, Contact Email, and Contact Phone.
So, for example, I made a sample post directly in the backend with "Yorkshire Tea" as the business, and "Headquarters" as the first office, with city = Harrogate, and dummy values for the contact email and phone fields.
I made a second business post, "RBS", and added a couple of sample offices.
So I now have two Business posts that I can import additional RFGs to.
I could make an initial import of more businesses, just a regular post import, but the interesting part is importing RFGs and connecting them to the relevant post, so I'll skip such a first import and will concentrate on what would be the second import, of the RFG data.
If you are not already aware, RFGs are stored in the database as one-to-many relationships.
So in my example above, "Yorkshire Tea" is saved in wp_posts as a business post type. "Headquarters" is also saved in wp_posts, as an "offices" post type, and the fields themselves are stored against that post in wp_postmeta. The connection between "Headquarters" and "Yorkshire Tea" is managed in the custom Toolset relationship tables.
So, on to my import to add some additional RFGs.
You can see in the screenshot "import sheet" the data I will be importing. It has the fields I need for each office, and the important part is the final column, which is the title of the business post they will get connected to.
So I create a new import with WP All Import Pro, to import the Offices post type, and the key part is shown in the second screenshot, where I have specified which column will populate which field. The post title (off-screen) came from the first column, the custom fields are populated by columns 2-4, and the final column which lists the business to connect the office posts to is added in the special section for Toolset relationships.
I completed the import and then checked the business posts in the backend, where I could see that the offices RFGs had been correctly imported.
Is that enough detail for you to be able to try the same?