Can't connect one particular website

This has been going on for over a YEAR. I do some updates for a client on their website, and suddenly, I cannot connect to the site on ANY browser on my iMac! I've had to use my iPad (which finds it without a hitch) to check it. It also works fine on my iPhone (8). This only happens to this ONE website.

I've looked for answers here, and none of the suggestions apply or work.

The weird thing is that it can't be the Router or modem, as since this started happening, I've moved to a NEW state, and now have a NEW ISP, a new router and new modem! My Wi-Fi iPad uses these devices and connects to the site without a problem. So does my iPhone.

But ANY browser I use on my iMac (running 10.10.5) (whether it be Safari, Firefox, or Chrome) will not connect to this ONE and only ONE website. It will show it trying to connect and then say something to the effect of a "time out" and "taking too long to respond."

I've run Onyx numerous times over the past year. Didn't work. I've reset permissions, SMC, PRAM, etc, etc, etc. I have changed my DNS to Google's and Open DNS (8.8.8.8, 8.8.4.4, 208.67.222.222, and 208.67.220.220).

Any thoughts?