Wordpress website not playing background video
Looking at the page source there is only one video tag on the above page and it does not include the autoplay attribute:
</div><video class="avia_video" controls id="player_12_811126710_1300552843" > <source src="https://www.kunstwinder.com/wp-content/uploads/2017/11/CarbonFiberRed_NoAudio_converted.mp4" type="video/mp4" /></video>Note you generally need to specify 'autoplay' and 'muted' to get the maximum possibility of autoplay at this time (the rules change from time to time).
Its also worth being aware that browsers may provide the ability for the user to override any autoplay instructions, via the browser settings or configuration.