I am being charged .99 monthly for the 50GB storage plan but my devices only show 16 GB of storage

It is impossible to increase the amount of internal storage in your iPad - as this is fixed during manufacture; what you have is fixed for the life of your device. 

You can, however, subscribe to a higher tier of iCloud storage - but this is not a substitute for internal storage.

About storage on your device and in iCloud – Apple Support

Manage your iCloud storage – Apple Support

If you have insufficient internal storage for your needs, you have two options:

• Manage the storage that you already have:

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

• Replace your iPad with a model with adequate storage for your needs.

iCloud storage does not add storage onto your device. It adds storage into Apple's cloud storage system, here you can save backups and photos.

For example, you could save space on your phone by keeping low quality/resolution photos on your phone but keep the high quality originals in iCloud.

Take a look at this help article for more info Set up and use iCloud Photos – Apple Support