Are the devices managed via AEDO? You might have some difficulty...

They are (I think but not sure what AEDO stands for).

Any further info?

Are SOTI in here? 

AEDO = Android-Enterprise  Device-Owner Mode  (use generic OEM-independent device agent and enrollment that requires device factory reset.  The active MDM api reported should include the item "Work Managed Device")

Generally speaking, with this management approach you would need to unenroll the devices from the EMM environment that they are enrolled into in order to disassociate them with the Device Owner, which is currently the SOTI agent. There isn't typically a mechanism at the device level to release this association as that would be a security risk for dedicated device deployments as it would be a vulnerability in the Device Owner architecture. This is one of the changes from the legacy world of Device Administrator where the end user could release that permission. Now, it is the Device Owner itself that has to relegate that permission off of the devices.

You might get lucky with a Factory Reset of all the devices but Factory Reset Protection might kick in, potentially re-enrolling the devices into the same EMM instance that they're already bound to. If you no longer have access to that EMM instance as a result of letting the licensing lapse then you may be in a tricky situation. 

Please note that the need to perform factory reset on AEDO devices when removing/changing MDM/EMM solution is not specific to Soti MobiControl, but is the same for any MDM/EMM solutions.  The mechanism is architectured by the platform owner (Google in this case), and the same is true for any secure iOS DEP/supervised devices for the Apple platform.