Hi,
Regarding your concern that adding your existing VM instance to your Manage Instance group is not possible, all we can do is add another VM instance by editing the Manage Instance group and add Minimum number of instances to create another one.
However, if you do not need the feature of Manage Instance group (stateless) there is another way to create HTTPs load balancer that will connect to your existing instance as backend service. You can follow the steps below.
- Go to instance group and click “New Unmanaged instance group”
- Create any name, make sure the location is also the same location with your existing instance.
- Network should be the same network with your existing vm instance
- On “select VMs” choose your existing VM instance. VM instance will not appear if location is different.
- Click create.
From the steps above, what we are going to use is an Unmanaged instance group so you can use your existing instance. Next is by creating a https load balancer. You will just follow the instruction on creating an https load balancer and from the backend configuration there is an option “instance group” just choose the unmanaged instance group that you created.
With regards to the managed certificate, I suggest configuring the setup that I suggest and wait for the DNS propagation. One troubleshooting step that you can do to check if the website is reachable is by accessing it using the external ip address of your website. Once it is accessible, double check your DNS configuration, especially the A record.