Sorry to hear that happened to you. They do lock out domains that were previously registered and still active from being re-registered [that's happened to me too]. Their Support should have offered you to contest and take over. It's not immediate [takes several days to go through the process and the departed employee will delay things] but if you can demonstrate positive DNS record control and go through the motions with support it does get done and you gain control. If they didn't offer it to you over support, try asking for it [if you are still wanting to go ahead with it]. It's not great speed but it also serves to protect you should someone else at the company decides to impersonate you and recreate the domain.
Best of luck!