WordPress Newsletter form issue (index.html and index.php)

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!" >

My issue is, that I have an index.html file as my landing page, as WordPress uses index.php. The newsletter form is on the HTML index. I have changed my form action to website.com/index.php?na=s

This just redirects back to the index.html version.

I know it is a long shot, but does anyone have any ideas what could be the issue here? I’ve tried asking a question on WordPress support, but their plugin support forum and contact form doesn’t seem to be working at the moment.