You cannot use iCloud Photo Library without WiFi. Furthermore, you cannot upload photos to iCloud for the purposes of backing them up. That is not how it works. iCloud Photo Library is a syncing service not storage in the sky. You are wasting your money by purchasing additional storage. You might want to think about canceling the upgrade.
Assuming you have a Windows Machine.
Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support
You can import photos to your PC by connecting your device to your computer and using Windows Photos app:
- Make sure that you have the latest version of iTunes on your PC. Importing photos to your PC requires iTunes 12.5.1 or later.
- Connect your iPhone, iPad, or iPod touch to your PC with a USB cable.
- You might need to unlock your iOS device using your passcode.
- You might also see a prompt on the iOS device asking you to Trust This Computer. Tap Trust or Allow to continue.
Then, follow the steps in these Microsoft knowledge base articles to learn how to import photos to Windows Photos app:
When you import videos from your iOS device to your PC, some might be rotated incorrectly in Windows Photos app. You can add these videos to iTunes to play them in the correct orientation.
Photos and videos that you sync from your computer to your iOS device using iTunes can't be imported back to your computer.
Learn what to do if you can't import photos from your iPhone, iPad, or iPod touch to your computer.