I think the pendulum's swung too far in the other direction: a proper mix of theory and practice is self-evidently the way to go--especially when immersion is not feasible--and the only part worth debating is exactly what mix to use. I never used Duolingo much because you have to type on a phone which is pure pain, but here's another reason I'd avoid it, and ultimately a more important one.

(Also, in case you haven't yet worked it out or looked it up somewhere else: you need to use "dem" with masculine and neuter indirect objects, often translatable into English using "to" or "for", and with certain prepositions. It is the dative masculine/neuter definite article.)