Hi there Tino Caer,
It sounds like you have tried removing and redownloading the songs that seem to get stuck when downloaded via cellular data. Thats where I would have started as well. I would next quit the Music, iTunes, and Settings apps:
Force an app to close on your iPhone, iPad, or iPod touch
1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.
When you have done that, restart the device and test the issue again:
Restart your iPhone, iPad, or iPod touch
If the issue persists I would next reset the network settings since that could also be causing issues here:
If you can‘t connect to a cellular network or cellular data
Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
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