The iPad storage is fixed during manufacture. It cannot be changed or upgraded short of entirely replacing the device with a model with a different storage capacity.
By contrast, your iCloud storage can be increased from the “free” 5GB (this usually being adequate to support basic iCloud functionality) to higher storage “tiers” by way of subscription. Unlike the built-in storage of your iPad, your subscribed iCloud storage can be increased or decreased as your needs change.
You may wish to review the reported storage of your iPad - as this may provide some clues as to how your local storage is being used by your iPad. With the benefit of additional information, it may be possible to provide appropriate advice.
If you can post a screenshot of the appropriate settings page here, it will help to inform the response.
Settings > General > iPad Storage
It will look something like this: