It would seem that you have lost the ability to file an NOL carryback because the 2017 return was not filed with 3 years of its due date including extensions under IRC 6511(d)
(2)Special period of limitation with respect to net operating loss or capital loss carrybacks
(A)Period of limitation
If the claim for credit or refund relates to an overpayment attributable to a net operating loss carryback or a capital loss carryback, in lieu of the 3-year period of limitation prescribed in subsection (a), the period shall be that period which ends 3 years after the time prescribed by law for filing the return (including extensions thereof) for the taxable year of the net operating loss or net capital loss which results in such carryback, or the period prescribed in subsection (c) in respect of such taxable year, whichever expires later. In the case of such a claim, the amount of the credit or refund may exceed the portion of the tax paid within the period provided in subsection (b)(2) or (c), whichever is applicable, to the extent of the amount of the overpayment attributable to such carryback.
(B)Applicable rules
(i)In general
If the allowance of a credit or refund of an overpayment of tax attributable to a net operating loss carryback or a capital loss carryback is otherwise prevented by the operation of any law or rule of law other than section 7122 (relating to compromises), such credit or refund may be allowed or made, if claim therefor is filed within the period provided in subparagraph (A) of this paragraph.
