Cannot mount vhdx file

Prior to installing Win11 26H2 I took the precaution of making a disk image of the current 24H2 on an external drive.
The 26H2 defaulted to a clean install so I lost everything on drive C: - most critically my Thunderbird profile and AutoCAD customisation files.
I want to reinstate 24H2 temporarily so that I can back up these to an external location (I have Synology NAS) then reinstall 26H2 and copy the saved files back.
However, when I try to mount the vhdx file from the image I get the notification 'The disk image isn't initialised, contains partitions that aren't recognisable, or contains volumes that haven't been assigned drive letters...'
When I attempt to use Disk Management to assign a new drive letter this option is greyed out.
Am I doing the right thing?
How can I reinstate 24H2 in another way?

How did you make this backup? Give details of the program or commands used.

I used Backup and Restore (Windows 7) to create an image on an external drive (E:)

I think you may be doing it wrong.

Method​

1. Open Settings → System → Recovery.
2. Beside Advanced startup, click Restart now.
3. Select Troubleshoot → Advanced options → System Image Recovery.
4. Select your Windows account and enter its password if requested.
5. Windows should detect the system image automatically if the backup drive is connected.
6. Select the image, then follow the restore wizard.