Jon
Parts of the config sound fine - as far as you have given us. I do not have a config to refer to but my memory of seeing ISR routers with the cellular card is that there was also a dialer interface configured. Is there a dialer interface in your config?
The static default route pointing to the cellular could be right, especially if you are not running a dynamic routing protocol. If there are 2 other connected networks they could use the cellular connection to get access out. Is there NAT or PAT configured using the cellular IP address?
You do not need a dynamic routing protocol as long as your static default route works and as long as the provider is routing back to you.
You certainly do need an ISP connection, but that is what the cellular card provides - as long as the cellular account is paid up and still active.
HTH
Rick
HTH
Rick