There are quite some things wrong with the iframe code you posted, some are minor but should still be corrected to ensure your code will work everywhere without issue.
First, do not use a semicolon after your source.
Second, you should not remove the https:// from the source url.
Third, and this is the reason it is not working for you, you did not copy the wmode="Opaque"
from the answer you linked.
So your iframe code should be:
<iframe width="560" height="315" src="https://youtube.com/embed/p0O1VVqRSK0?wmode=transparent" wmode="Opaque" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
See this fiddle for demonstation:
https://jsfiddle.net/7ox49Ldv/3/
If it is still not working for you with the corrected iframe code, there must be a deeper issue with your html/css.