Hello pawled,
You can try adding the class .btn-floating and inline style border-radius: 0;
For example:

  <a
   class="btn btn-primary btn-lg btn-floating"
   style="background-color: #ac2bac; border-radius: 0;"
   href="#!"
   role="button">
    <i class="fab fa-instagram">
    </i>
</a>

If this solution does not solve your issue please send more details.
Best regards