Gosh, there are some astounding responses from retailers out there when it comes to DEP.
Steer clear of the ones that either: don't know what DEP is, or, can't retrospectively sort out DEP for you, or say that you need to be on a specific mobile carrier to get DEP!??! WTF?!
Josh has nailed it... Apple finally saw the error of their ways and have allowed for what they call "Provisional DEP Enrolment" using Apple Configurator 2.5 and iOS 11.
The only caveat is that the provisional enrolment can be removed by anyone for up to 30 days.
Therefore you may need to leave your newly enrolled devices in a cupboard for a month, so that a little darling doesn't remove the profile!
Ultimately tho, find a good reseller that knows DEP properly and you wont wind up in a world of pain.
Oh and MAKE SURE, you provide the correct DEP ID when you purchase and that your reseller does the same (cough cough) or else that gets VERY MESSY too!! 😱
Josh and I are your guys if you need any Apple assistance... we've seen the good, the bad and the plainly-defies-all-sense-and-reason-dammmmnn-ugly... so, please just hit us up!