Disk has multiple partitions but Disk Utility can only mount first one View from Disk Utility:


Output of diskutil (for this disk only):

/dev/disk6 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *80.0 GB    disk6
   1:               Windows_NTFS                         34.9 GB    disk6s1
   2:                      Linux                         10.7 GB    disk6s5
   3:                 Linux_Swap                         2.1 GB     disk6s6
   4:                      Linux                         32.2 GB    disk6s7


The first NTFS partition, "--", mounts just fine as read-only. But I can't get to either of the Linux partitions.
I've tried mount, diskutil mount, and diskutil mount readOnly, but but they all result in 

Volume on disk6s5 failed to mount
If the volume is damaged, try the "readOnly" option
If the volume is an APFS Volume, try the "diskutil apfs unlockVolume" verb


Also tried diskutil apfs unlockVolume, but it's not an APFS volume. Any ideas? Using macOS Sierra 10.12.6.
macOS can't (actually won't - it could), it used to be possible with simple add-ons, but SIP has made most of these unusable.

There are paid options, but not sure if these are still usable with recent macOS.

You can read ext4 with a Linux Os, either a virtual machine, or boot from a live CD or image.