Why can’t I update my software with my cellular data
They said I cannot update my software with my cellular data unless I use WiFi
You didn't state the model of your device and didn't explain who "they" are. If you received a message that you need to use Wi-Fi then it would seem you cannot use phone data.
For iPhone, Apple says in Use 5G with your iPhone that you would need an iPhone 12 or later to use 5G to update your phone with data and only if your plan supports high data use.