Expired certificate

Hoping someone can help.

I've recently wiped my 2014 MacBook Air and have been unable to use it, as every time I try to setup Next DNS by visiting the webpage, whatever browser I'm using (preference for Chrome) I'm told the site isn't secure. The certificate seems to be valid with almost 2 months left and my computer's date and time is set automatically. I'm not great at this sort of stuff, my friend helped set it up after my network was compromised and neither of us know what to do.