Other than putting the earlier version on a USB drive and trying that way, but you might have tried that already, and the 750 might not recognise it as being an "update" (seeing as it's older firmware).
Otherwise, the only thing you can do is call customer support, and if they can't help you, I would return the unit, honestly, that's what I'd do (there are better extenders out on the market now that will definitely perform better).