How to force a re-association with UTM that was accidentally deleted from a SUM

We have a UTM that was deleted from our SUM. Other than this, nothing has changed on the SUM or the UTM, but the UTM is not showing up in the SUM. 

• The UTM is still configured to connect to the SUM.
• The UTM is not in the "Deny" section on the SUM.
• On the UTM under Central Management, says that since MSP licensing is enabled, changes cannot be made on that tab.

So... how do I get the UTM back into the SUM?

Thanks,

Peter