First thing you want to do is make another copy of your .dmg file to keep untouched in case any utilities you use to access the image makes the problem worse and you need to start fresh again. Always work from a copy and keep the original safe.
Did you test the .dmg file after you created it? Are you trying to open the .dmg file from an external drive or the internal drive? Try accessing it from a different drive and another Mac if you have one.
I have no idea if any of this will work especially with Mojave especially if the .dmg contains an APFS volume:
1) TestDisk -- should be able to work with an image file
2) 7zip -- may be able to provide more information about the issue even if it fails to open it.
I don't know if any of the commercial utility software is able to operate on a .dmg image file or not. Utilities I've used on standard HFS+ volumes in the past include Disk Warrior and Data Rescue, but they won't work if the .dmg contains an APFS volume.
You may need to contact a professional data recovery service to see if they can assist you with a broken .dmg file if there are no other suggestions.
