Failing to send a periodic statement means failing to disclose the APR and FC for these billing cycles. These serial violations have created an unknown amount of civil liability and expose you to enforcement action by your regulator.
While there is no cookie-cutter solution to open-end credit reimbursements, the "Joint Statement of Policy on the Administrative Enforcement of the Truth in Lending Act—Restitution" clearly applies. Since there have been very few open-end credit reimbursement cases since the Policy went into effect in 1980, there's no way to know what remedial action your regulator might order you to take. Because you found this problem, the "Policy" does not apply (yet.)
Section 130(b) of the TILA (15 U.S.C. § 1640(b))
offers a voluntary
self-cure, but taking this cure would mean reimbursing 100% of the undisclosed interest for all billing cycles going back 2 years.
For now, taking no action is an option. Should your regulator discover this problem during a future compliance examination and invoke the "Policy", it's hard to imagine how the cure could be worse than 100% of the interest collected without benefit of legal periodic disclosures.