Form button not centering

I am having a problem with the submit button centering....

<form id="myform" class="contact-form" action="contact.php" method="post" >
                <div class="row">
                    
                    <div class="mymail">
                        <input type="text" name="name" value="" maxlength="100" placeholder="Name"/>
                    </div>
                    
                    <div class="mymail">
                        <input type="text" name="postal_code" value="" maxlength="10" placeholder="Postal Code"/>
                    </div>
                    <div class="mymail">
                        <input type="text" name="email_address" value="" maxlength="100" placeholder="E-Mail"/>
                    </div>
                    <div class="divider" style="height: 50px"></div>
                    <div class="col-md-12">                        
                        <input class="sub-btn p-btn-medium" type="submit"></input>
                    </div>
                </div>
            </form>

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.sub-btn,
.p-btn {
    margin-right: auto;
    margin-left: auto;
	display: inline-block;
    font-size: 15px;
    line-height: 25px;
    letter-spacing: 0.5px;
    font-weight: 300;
    padding: 6px 25px;
    border: 1px solid #3c3c3c;
}

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David SchureAsked:
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David S.Commented:
Auto margins only center block-level elements (those with an outer display type of block). Inline-blocks have an inner type of block but an outer one of inline. I suggest giving a "buttons" class to its parent and giving "text-align:center" to ".buttons" in your stylesheet.

Also <input> element are always empty and so should not have an end tag.  I recommend using the <button> element for buttons in forms instead of the <input> element.
                    <div class="col-md-12 buttons">                        
                        <button class="sub-btn p-btn-medium" type="submit">Submit</button>
                    </div>

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RobOwner (Aidellio)Commented:
change the display: inline-block to display: block;
David SchureAuthor Commented:
Thank you David S.
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