On September 15th equipment rental fees earned during the first 15 days of September amounted to $6,100, which $5,300 was received in cash.

Would this be the correct entry?

Debit cash for the 5,300 and accounts receivable for the 800 and then credit rental fees earned for the 6,100 or would that also be unearned rental fees.

This one wasn't paid in advance. They paid the fees for the first half of the current month. Since that is being provided now, it's being earned now.