This is simply a bad idea to continue to use this, why?
1) The two drives shown in raid management are displayed as /dev/sdb3 and dev/sdc3, the 3 defines a partition, OMV does not use partitions to create arrays it uses the 'full block device' (the whole drive). Whilst an array can be created on the cli using partition/s the problem comes when trying to replace a failed drive/partition within an array
2) AFAIK DSM uses storage pools then volumes before creating an array, that can either be hybrid or a standard array Raid0, 1, 5, 6, etc
3) Continuing using this would initially be OK, until a drive fails and requires replacing, you're than into another can of worms trying to replace a drive within the array that was not created in OMV's GUI
4) Put the drives back in the Synology, back up the data you cannot afford to lose, then add the drives back to OMV, secure wipe them and create an array within OMV's GUI