I've already answered this on our call, but I think it's a good idea to share this because others may find it useful.
This is the data model we're working with:
As a result, in order to be able to load NLR Timephase, one first has to load NLR link. This is necessary, since each NLR can be linked to many work items. When loading NLR Timephase, NLR Link that was created in advance must be populated in the RelatedLink column on the file you're importing.