Connect Safe and Browser Insecurity

Been using the ConnectSafe DNS for several years.  Problem is, while I can block the Porn/Mature sites with the appropriate DNS entry on my router; this does not suppress Porn images from surfacing under a Bing, Safari, Google search etc..

I've been told the only way is to increase the settings in the browser to block explicit/adult images.  But, the challenge with this is that I have too many devices (IOT) that have embedded browsers and many of these cannot be set.  

Also, my teenage friends desire to use the "guest" account through my router and I have no control over their devices to reset their unique browsers (Chrome or Safari).

Is there anyway that the DNS can be modified to block the browser searches and returned cached images; or, is there a way to block the explicit material via the router.  Been trying to resolve this one dilemma for over a year plus without nil success.

/s/ Conrad A.N.