Steps to configure the right certificate if it is not on the Default Web site!

You can use the same certificate which you have configured on the FE server for MTLS. Make sure that the Server name is there in the SAN or as a subject name.

Check if you can see the address book files under the virtual directory Default Web site => Abs => Int => Files as follows

Check if you can see the address book files under the virtual directory Default Web site => Abs => Int => Files as follows

The output location in above figure should match the location of local path in the following figure. and the "File share location for internal connections" should point to the web server name.

Locate abserver.exe on the address book server and run "abserver -syncnow" command if everything is fine. It will generate the latest copy of address book and the clients should be able to download the same.