Registered Inseego FX2000

I have an Inseego FX2000 that I'm trying to decommission for a customer so they can repurpose it. I'm not sure what the process is to unregister the device from T-Mobile so the next person who uses doesn't have any issues. I'm not sure if there is anything they have to do or not with T-Mobile? I tried to register it through Inseego and the error I received was "Unable to register, please contact Inseego Customer Care team." When I contacted Inseego they said I needed to contact T-Mobile. What needs to be done so the next person who uses doesn't run into any issues.