You can use CSV Import to import relations between resources.
When you say it didn’t work, what did happen? An error? Any messages in the job log for the import? Nothing imported at all or just different from what you expected?
Well, in the “related” field, I have the numeric value (let’s say: 4) which is the exact id of the linked resource. In my resource template, I have also set that property “related” is of type resource item. However, when I import the CSV, it does not “understand” that this is a relation and just adds the resource with a key value pair (related: 4). No linked resource.
I think the issue here is just that you haven’t selected the right data type for the column when importing it. You set that using the “wrench” icon for the column in the mapping table, and that will open a sidebar where you can choose what data type to import that column as.