Thanks to all who answered.
Threadi was right about the RevSlider plugin when deactivated the site corrected itself.
The author of the theme doesn’t agree, they think it the AIOSEO plugin at fault with their theme so they are going to change the theme accordingly. Currently the website has RevSlider plugin activated and AIOSEO deactivated
Thank you @codersantosh I unticked the option and behold H1 appeared!
