I've never tried the DA->DO migration process myself as we pre-emptively put most of our customers on Android Enterprise before we were forced into it. The Atlas devices like the TC56 were certainly in that transition period where you had to make the decision to move them over to AE early while they were on M or N, so that you could be future proofed for when O arrived. In the few scenarios where we deployed AOSP M or N Atlas devices on DA, we eventually just factory reset them and upgraded them to O GMS before re-enrolling on AEDO since this new process was still theoretical at the time. 

I'm still curious to see if and how this actually performs however. I'm impressed that its worked all the way to the point of being in Work Managed mode as that at least confirms you've migrated from DA to DO. It does appear like you have met all the pre-requisites. Some additional questions come to mind:

• Do devices have access to the Play servers from the network that they're operating on?
• Have you waited long enough? Sometimes the account assignment can take some time.
• Are the devices on a GMS build? (They appear to be based on the NG distinction)
• Does the Reset Account option work?

• Do devices have access to the Play servers from the network that they're operating on? Yes these devices use SIM cards for connectivity so no wireless 
• Have you waited long enough? Sometimes the account assignment can take some time? Yeah i have waited 
• Are the devices on a GMS build? (They appear to be based on the NG distinction) Yes the devices are GMS
• Does the Reset Account option work? I actually upgraded the server to make use of this feature but get the below