Recovering time machine backup on failed external disk partition

I have an external disk with apfs filesystem that contains my encrypted time machine backups. The filesystem get corrupted and I'm not able to mount it any more. Disk Doctor is not able to fix the issue. I also tried to run fsck_apfs in terminal, but I guess this is the same. I always get an error: The volume /dev/disk3s2 could not be verified completely. I suspect that this is not a hardware failure as I checked with WD Drive Utilities. Is there any other tool that can fix the filesystem?