Hi,
This is all the knowledge I have, try these steps. Unfortunately if this does not work, there's nothing I can help you with.
Make sure you have mdadm and lvm2 packages installed in your Ubuntu Linux.
For my test, I did an installation of Ubuntu and made sure NOT to use Logical Volume Management so that I was sure that my tests only show results of the synology volume. Don't know how bootable USBs show up, but I guess that will not be an issue.
FIRST OF: become and stay(!) root by typing "sudo su -" (don't forget that last minus-sign) and create a mount point you can use by creating a directory:
mkdir /mnt/syno
mdadm --detail --scan
Try this, see if your RAID details are visible
fdisk -l
Check if you see a mdxxx device. In my test, I saw a /dev/md127. Try to mount it, create a mount point somewhere (remember that mkdir /mnt/syno???) and try to mount: "mount /dev/mdxxx /mnt/syno".
But probably you will get an error (unknown filesystem type 'LVM2_member')
lvmdiskscan
This will output all kinds of info. You are hoping to see something like /dev/vg1000/lv and-or /dev/mdxxx and "1 LVM physical volume"
lvdisplay
You should see something like "LV Path /dev/vg1000/lv".
vgdisplay
Should display the volume name
lvscan
Should return the volume of your synology. When you see "Active", you are as good as set. If you see your volume, but "INACTIVE",
try "modprobe dm-mod" followed by "vgchange -ay" and run lvscan again.
If it's ACTIVE (for example, you see the output "ACTIVE '/dev/vg1000/lv' [xxxx GiB] inherit" you should be able to mount it with
mount /dev/vg1000/lv /mnt/syno
That's all the info I can give you. I wish you all the luck!