Pali correctly indicated that perhaps said certificate has expired. So you may have to just check with your ISP.

However, I also get that error with my own WPA-Enterprise network with self-signed certificate, when the root is not on device. Adding it causes the error to not come up anymore.

In your case, if certificate is not expired, you still going to need to contact your ISP for their root cert

They may have it on some website of theirs or something actually ..