Secure websites not loading on Mac in Safari and Chrome

I am having problems with secure websites not loading all of a sudden. I have several secure sites that I use for work and a couple of them have suddenly stopped loading on my Mac. I can use them on my Mac-book Pro or my phone, but not on my Mac. Not all of them, only some of them. I find it happening on both Safari and Chrome. I primarily have to use Chrome for work and a few sites in Safari are either extremely slow to respond or don't completely load.

I've tried clearing the histories, forgetting my Network Wifi then resetting it, using my iPhone Hotspot. Nothing is working. The Wifi is working on all other devices just fine, so it makes me believe it is my computer, but it's only a few websites.

I saw something that said to change DNS settings, but I have no idea what DNS is or why or how to change them. What is the next best step to try?

Thank you!

DJ