I said that the field you are searching and the input you are using need to be of the same type. Not the RG itself.
So the RG is set to invoice. That’s is fine. But the searchbox input to search for customer will be set as type Customer. Display of searchbox will be name. And the filter in RG will be For customer = Searbhox input value
For customer is a “customer” type and the searchbox will also be of type “Customer”.
You can select ignore empty constraint, so it will just ignore this filter if nothing is selected in Searchbox.
You can add more searchbox or input text to filter differently. I don’t use fuzzy search but It’s a little bit more flexible to search multiple fields at the same time on one type. But you will face the same limitation is that you cannot search fields in a related DB directly in the search filter. Only possible using :filter and advanced filter functions