Sometimes we may play videos without wifi or even without connection at all (for kids), we can use Google Drive or other apps like Dropbox to store these videos, and downland them as offline.
Check Mobile Data Usage
In Android, search “App data usage” or Go to Settings -> Network & Internet -> Mobile network -> App data usage
We can also disable “background data” for specific apps.
In iPhone, go to Settings > Cellular.
We can also disable “background app refresh” for specific apps.
User other’s phone
Use someone else (family member)’s phone who has an unlimited data plan like corporate plan
Misc Tips
Avoid send video files or big images without wifi.