Thank you for using Apple Support Communities! I understand that #images was not working as expected on your iPhone over cellular data, and you are also unable to add it back to your app drawer. You've come to the right place for help.
Let's start by trying to add #images back to your app drawer following the steps here:
Send and save GIFs on your iPhone, iPad, or iPod touch - Apple Support
Specifically, you'll want to look for it under 'More' instead of the Messages App Store:
If #images is missing from app drawer
Make sure that the #images app is enabled:
Make sure that your keyboard is set to a supported language and region. Go to Settings > General and tap Language & Region.
The #images app is available in the United States, United Kingdom, Canada, Australia, New Zealand, Ireland, India, Singapore, Philippines, and Japan.
Once you are able to add #images back to your app drawer, check that cellular data has been enabled for the app as well from Settings > Cellular > HashtagImages. Let us know what happens from here.
Cheers!