Router Admin Tool Is Not Secure

The business I do IT work for got a new Xfinity modem on 4/13/2023.  We called for a technician to visit because the internet connection was going in and out.  He replaced the existing Xfinity modem with a new one on that date.

I enabled remote management.  I set the protocol to HTTPS and changed the default port number to another one for added security.  Both Chrome and Edge browsers show that the host is not secure, and will only allow HTTP, which causes the modem to reject logging in.

I spoke by Chat with Comcast support about the issue.  The agent told me there's no such thing as remote management and refused to assist me any further.

I then called the Comcast business support number, which always forces you to needlessly accept a reboot of the modem, and needlessly disrupt your internet connect, before you can speak to a tech about anything at all.  So after wasting my time with this, I finally got to request a call-back from a tech.

The agent I spoke to advised me to set the firewall level for IPv4 to a level higher than the default low setting.  I tried this, and it did not resolve the issue.  I remote using the public  static IP address with HTTPS and the port number I assigned.  If I try the IP address the settings page in the modem says to use for remote management, it simply redirects to the public static IP address for the gateway, and the web browser shows the host is not secure.

The old modem we had did not have this issue.  After I set up remote management it worked fine until a couple of months ago, when it started exhibiting the same behavior as the new modem.

I wonder if anyone can tell me what I'm doing wrong or missing here.