It looks like ShipStation is importing only orders from the last month. If I want to import older orders, I must contact the ShipStation customer service so they do it manually on their end.
The issue is that I'm unable to get in touch with anyone from ShipStation. They were not available over the weekend, which is understandable, but now it's almost 1pm on Monday, and still my ticked is unassigned.
(Case ID: 109271)