In the absence of a time for payment, payment is due in a reasonable time. What is reasonable depends on all the factors of the relationship including things like prior dealings, industry norms, the nature of the work etc.
The OP has stated in a comment that the invoices were due “immediately” - this is not reasonable for a “freelancer” arrangement. Between 7 and 60 days is more likely the bounds of “reasonable”.
In the absence of a contractual provision, the contractor is not entitled to late fees or interest, particularly where it is unclear what is late and what isn’t.
If the non-payment persisted beyond what was reasonable the contractor could sue and the court would determine a definite date on which payment was due and the judgement would create a debt. The contractor would be entitled to interest on the judgement debt because that is a statutory rather than a contractual entitlement.