Thanks Zoltan for your reply and the advice.
I waited for the Nokia 6.2 to be released and a good offer which I found on Black Friday. So, I’d like to update the thread in case someone is looking for a USB-C cable. I use the phone with the following cable: https://www.amazon.com/gp/product/B01GGKYT7M
I can confirm that this combination works well with the Nikon Z7. So, whoever needs a USB-C cable for the Z7, this one works. I also used a USB-C cable delivered with a power bank and it worked as well, so maybe USB-C is less demanding in terms of OTG.
I just noticed that the RAW pictures of the Z7 can't be displayed in the app. I see the list of pictures and the filenames (so the recognition of the files itself works), but the pictures are just black (on the camera screen, they are fine). Would be nice to see the pictures after capture to quickly check them without the need to use the camera display. Any ideas?
Oh, and one completely different question: As mentioned in my first post, I'm an iPhone user and I bought the app on iOS as well. After Apple removed the app again, I can't install it on my iPad (don't see the app in purchases) and I'm worried that after buying a new phone or resetting the old one, I won't be able to restore my purchase there as well.