This must be configured along with the mobile data limit.
As the router cannot understand whether your SIM card emptied in credit or it just stopped working, you must configure something that it would use as an indication for the size of the data and period of time that it's renewed.
Go to Network > Mobile > Mobile Data Limit and there you will be able to set the limit and the warning about it. So if you want to only know when your card emptied in data - just put the number of how much are you using / buying. And in the warning do the same thing, but minus like 1 or 2 megabytes in that field, in case your SIM does not have any money left for SMS.