Safari cannot find server, using any device

You effectively ruled out any problem with your Macs or your iPads, which leads to the logical conclusion the problem lies with your router or your ISP.

The site is loading ok on the Mac I happen to be using at the moment, so if you were to contact them they would probably tell you everything is ok on their end also. Try resetting your router — power it down, let it rest an arbitrary minute or so, power it up again, test.

Sure, I understand. It could be an odd setting in your router, or it could be that your ISP is having trouble with its DNS server. Nothing you can do about the latter other than to contact them about it.

For what it's worth here is that website's "contact us" page (click to enlarge):

There is no email address as such; the "email us" link is just a popup form to fill out. I'll save you the time though... they will just tell you to try everything you already tried.