The Verdict: Need More Info
Having been Thai yourself, you may be able to resume nationality, or at least naturalise without the residence and language tests. Take our reacquisition quiz, which turns on how and when you lost it.
Section 23 lets a woman who renounced to take a husband's nationality resume once that marriage ends. Section 24 lets someone who lost it as a child resume within two years of turning 20.
Otherwise, section 11 waives the five years' domicile and the language test for former nationals, which removes the need for permanent residence.
Find the Royal Gazette entry recording the loss, which proves you were Thai and dates it. If nothing was ever published, you may still be Thai.