HTML form on external page not working

Hi there,

I have added my HTML signup form on an index.html page in my main root WordPress directory, but the form is not adding emails to my database.

This is the code I am using.

<div class="tnp tnp-subscription">
<form method="post" action="https://www.website.com/?na=s" onsubmit="return newsletter_check(this)">
<input type="hidden" name="nlang" value="">
<div class="tnp-field tnp-field-firstname"><label>First name or full name</label><input class="tnp-firstname" type="text" name="nn" ></div>
<div class="tnp-field tnp-field-email"><label>Email</label><input class="tnp-email" type="email" name="ne" required></div>
<div class="tnp-field tnp-field-privacy"><label><input type="checkbox" name="ny" required class="tnp-privacy">?By continuing, you accept the privacy policy</label></div>
<div class="tnp-field tnp-field-button"><input class="tnp-submit" type="submit" value="Yes, keep me informed!" >
</div>
</form>
</div>

Can anyone suggest what I have wrong?

Many thanks!