The cellular module check tool is checking through Alarm.com itself, and is not specific to suretyDIY. There are a couple different responses that might indicate a module cannot be used. What is the specific response you are seeing?
Also, to clarify, if you have a current service provider, as in your cellular module is tied to an active Alarm.com account, the check tool will report that it is already assigned to a unit. This is normal.
Each communication module can only be assigned to one Alarm.com account at a time. In order to set up a new account with it, the old account must be cancelled.