@jared_f I could but when the device is transferred over to the new network it removes all of the apps from their home screens (something to do with the dodgy white list on the old network) and they have to sign in to their email accounts again so I had just been moving devices over as and when they come through the IT department or when we have a problem.
Is there somewhere I can regenerate the QR code for the correct network?
Thanks
UPADTE: I have just scanned the v7 - v8 (Oreo) QR codes for both the old and new networks (Systems Manager > MDM > Add Devices) with a barcode scanner app and they are producing different URLs so its strange they they end up enrolling into the same network! - I am going to test some more with a new device now...
UPDATE 2: After enrolling a new device the QR code has worked, must have take some time for the QR code to update when I change the default network from this morning.
@Melissa's solution was the fix, it just took time to apply the change and update the QR code.
Thanks for your help everyone!