Not sure if this helps in your particular circumstances, but here are the instructions:

https://support.duolingo.com/hc/en-us/articles/115004068103-How-do-I-download-lessons-

There are some other Plus related articles that you may also want to check out:

https://support.duolingo.com/hc/en-us/sections/115001321983-Duolingo-Plus

Well, you have finished your German tree.

So the biggest advantage of Plus is gone, that you can download locked (grey) lessons with the blue Plus symbol (in the tree per skill).

If you go to any completed skill, you will see a white Plus cloud.
If you click on it, it should download THIS SPECIFIC skill for offline usage.

1) AFAIK you can re-do the lessons for a skill.

2) AFAIK you can NOT do strengthen exercises. Please try and report.

3) AFAIK you can NOT download the full course for offline-usage with a single button click (like Memrise Pro on the Android app).

I have already seen the basic (quite limited) Plus functionality symbols on the Android app on Bluestacks emulator, where you can download 3+2 or 2+3 skills all at once.

I've checked again/done another level. Despite just paying for the subscription and having the confirmation, it is still saying i havent subscribed.