Accessing unmountable APFS Volume on a disk image
Welcome to Ask Different. Maybe you can mount the APFS volume by using the 3rd party app Mountain. Others have had good luck using this app. The app is not free but has a free trial. It's easy to download and give it a shot. I'm not associated in any way to Mountain. I'm just a happy user. The app requires macOS 10.8 or later.