Extension Study Evaluating NTX100 Neuromodulation System for Medication-Refractory Primary RLS

Inclusion Criteria: 1. Subject previously completed the RESTFUL Study (NCT04874155). 2. Subject has signed a valid, Institutional Review Board-approved informed consent form, can understand the requirements of the study and instructions for device usage, and can converse in English. 3. \[applicable to Arm 1 only\] Subject agrees to not change dosage or schedule of any medications that are known to impact RLS symptoms during the study, including RLS medications, antidepressants, sleep medications, or sedative antihistamines. Exclusion Criteria: 1. \[applicable to Arm 1 only\] Subject has active medical device implant anywhere in the body (including but not limited to pacemakers, spinal cord stimulators, deep brain stimulators) or metal implant at the site of study device electrode application. 2. \[applicable to Arm 1 only\] Subject has been diagnosed with one of the following conditions: * Epilepsy or other seizure disorder Current, active or acute or chronic infection other than common cold \[and except for acute infections with mild symptoms\] * A malignancy within the past 5 years (not including basal or squamous cell skin cancer) * Stage 4-5 chronic kidney disease or renal failure * Severe movement disorder symptoms (Parkinson's disease, Huntington's disease, dyskinesia, dystonia) * Deep vein thrombosis * Multiple sclerosis 3. Subject has moderate or severe cognitive disorder or mental illness. 4. \[applicable to Arm 1 only\] Subject has known allergy to device materials, electrode gel, polyurethane foam, or lycra (or severe previous reaction to medical adhesives or bandages). 5. \[applicable to Arm 1 only\] Subject has severe edema affecting lower legs. 6. \[applicable to Arm 1 only\] Subject has any of the following at or near the location of device application. * Acute injury * Cellulitis * Open sores * Other skin condition 7. \[applicable to Arm 1 only\] Subject is on dialysis or anticipated to start dialysis while participating in the study. 8. \[applicable to Arm 1 only\] Subject has received another investigational device or drug within 30 days before study entry, is planning to receive another investigational device or drug during the study, or is planning to change RLS medications during the study. 9. Subject is unable or unwilling to comply with study requirements. 10. \[applicable to Arm 1 only\] Subject is pregnant or trying to become pregnant. 11. Subject has a medical condition not listed above that may affect validity of the study as determined by the investigator. 12. \[applicable to Arm 1 only\] Subject has a medical condition not listed above that may put the subject at risk as determined by the investigator. 13. Subject was unwilling or unable to follow instructions in the RESTFUL Study, including missing 2 or more follow-up evaluations. 14. \[applicable to Arm 1 only\] Subject failed to properly operate the investigational devices during Weeks 5-8 of the RESTFUL Study. 15. \[applicable to Arm 1 only\] Subject experienced a significantly higher than expected rate of device malfunctions in the RESTFUL Study, such that the malfunctions appeared to be directly related to the subject's pattern of use.