We have 2 zonedirectors ZD302, one is being used as standy backup. We renewed our certificate and at first everything seemed fine. But when people login to our guest wifi and when they are being redirected to the portal website, they get the message that the website is not safe and the certificate is invalid. But when I type in the guest portal url in the browser of my laptop, I can access it just fine.
What is the problem here?
Most probably you don't have full certificate chain installed. Some browsers may download missing intermediate certificates automatically, and other -- not. Your PC most probably have it already cached.
If you miss intermediate certificate, browser will still not accept public certificate as valid, as it can't check it validity until imported CA certificates it trusts. You can use on-line tools to check this, if portal can be accessed from Internet -- https://www.supermonitoring.com/blog/free-tools-to-check-ssl-certificate/ . But probably it can't, so you need to check in browser manually in detail, what is missing.