A subject must meet all the following criteria to be eligible for inclusion in this study: 1. History of confirmed diagnosis of SARS-CoV-2 infection or COVID-19 per section 9.4. 2. PCC (per protocol definition) with cough and at least 2 of the 3 additional qualifying symptoms that have been determined not associated with another disease state: - fatigue - shortness of breath - cognitive dysfunction/brain fog. 3. Negative for COVID-19 via rapid antigen test. 4. Minimum time period from onset of symptoms (or from date of positive test for asymptomatic) - 3 months. 5. Minimum duration of symptoms at least 2 months, continuous or intermittent. 6. Males or females, 22 years of age and older on the date of enrollment. 7. Provides written informed consent prior to initiation of any study procedures. 8. Be able to understand and agrees to fully comply with defined and described study procedures and be available for all study visits for the entire study duration. 9. Agrees to perform self-treatment twice a day, separated by ≥ 6 hours during the treatment period. 10. Agrees to maintain current medications without the addition of any new medications for relief of signs and symptoms of PCC during the entire study period, and, if used, to report ALL such medications (including over the counter and home remedies) to the study staff. 11. No uncontrolled disease process (chronic or acute) that could be attributable to qualifying symptoms. 12. No physical or mental conditions or attributes at the time of screening, which in the opinion of the PI, will prevent full adherence to, and completion of, the protocol. 13. For women of childbearing potential, a negative urine pregnancy test at screening. Subjects (female subjects of childbearing potential and male subjects with partners of childbearing potential) must agree to use proper** contraceptive methods to avoid pregnancy during the study.

A subject with any of the following criteria will be excluded from participation in this study: 1. Positive test for SARS-CoV-2 infection or COVID-19 within the past 30 days. 2. Individuals < age 22 at consent. 3. Any medical disease or condition that, in the opinion of the site Principal Investigator (PI) or appropriate sub-investigator, precludes study participation. 4. History of recurrent alcohol intoxication or other recreational drug use (excluding medically prescribed cannabis) within 30 days of study day, V1. 5. History of any systemic antiviral therapies within 30 days of study day, V1. 6. History of oral or parenteral corticosteroid use within 30 days of study day, V1. Active use of nasal or inhalable corticosteroids is also exclusionary. Topical steroids are not exclusionary. 7. History of any chronic medical condition that has required adjustments to the type, dose, or schedule of medical treatments within 30 days of study day, V1. 8. Requirement to use narcotic medication for analgesia. 9. History of hypersensitivity or severe allergic reaction (e.g., anaphylaxis, generalized urticaria, angioedema, other significant reaction) to sun exposure. 10. Presence of any oral abnormality (e.g., including, but not limited to, ulcer, oral candidiasis, oral mucositis, gingivitis, history of frequent recurrent aphthous ulcers, burning mouth syndrome, dry mouth syndrome, a disease that can result in xerostomia (e.g., Sjogren's syndrome), or other oral disorder that in the opinion of the investigator would interfere with device use and evaluation. 11. Any intra-oral metal body piercings that cannot be removed for the duration of the study. Metal orthodontia is permitted as braces will be covered by the device mouthpiece. 12. Any individual without teeth or with a dental malformation that precludes directed use of the device as intended. 13. Use of new medications to treat PCC symptoms within the 30 days prior to V1. 14. Currently enrolled in or plans to participate in another clinical trial with a therapeutic investigational agent for any medical indication that will be received during the study period. 15. Has participated in an investigational treatment trial for PCC within the past 30 days. 16. Subjects with immunodeficiency syndromes, including HIV, per medical history diagnosed prior to previous acute COVID-19. 17. Subjects who have previously participated in RD-X19 acute studies, P20 or P30. 18. Is pregnant or nursing, is not post-menopausal, or is not currently on birth control. The rhythm method is not an acceptable form of birth control for this study.