What can I do to fix “cellular update failed”?

The "Related Article" above your question has all the info. Contact Apple Support

Or

If you can visit a nearby Apple Store, you might be able to use a computer there.

(Best to phone them first, for a reservation:)

or

Phone Apple Support at 1-800-APLCAR

Else:

See if this article can help:

If your iPhone or iPad won't update - Apple Support

All the best :-)