VA's Duty to Assist assists you in getting evidence, such as medical records, to help support your case and only applies during your initial claim and if you file a Supplemental Claim.
Are you going to request  Higher-Level Review or Board Appeal?
Yes,  am going to request a Higher-Level Review.
Your Duty to Assist will not apply; but the reviewer or judge will make sure the VA met its Duty to Assist during your initial claim.