Bad xfinity dns certificate and 404 error on specific site

I receive a certificate error:
Low-xdns.xfinity.com
expired august of 2019
when trying to connect to
 https://02b5896.netsolhost.com/fileman/fileman.cgi?
clicking 'proceed to web site' fails with a 404 error.
I contacted the hosting provider of the site and on other networks the site IS AVAILABLE.

I have tried the chat assistant, looking in forums, calling and I am about fed up with Xfinity. This has been going on for years. I literally haven't been able to update my page since 2016/2017.