This app was mentioned in 3 comments, with an average of 1.33 upvotes
I have used duolingo before but I still have trouble remembering the articles. I just published a free german article quiz app for android that allows you to practice your knowledge of the german nouns. https://play.google.com/store/apps/details?id=com.londonappstudio.germanarticles
Nothing major but after I started learning I made a little German Articles Quiz app for Android which I originally thought would be helpful for my own learning and then published it on the Google Play store a few years ago. At its peak I made a bit of money off the ad revenue. It wasn't a lot, but was nice to get something back for the effort of making it, and getting any positive feedback always feels very rewarding too. Now it's popularity has fallen massively due to lack of updates, and other shiny new apps being published, so the ad revenue is almost nothing.
I'm hoping you're an Android user... I recently published a free german article quiz app for Android that has tips for learning these. It also allows you to practice your knowledge of common german noun genders. https://play.google.com/store/apps/details?id=com.londonappstudio.germanarticles It would be great if you could try it out and let me know if you find it helpful.