It is a 12.1 subscription version. I only have the licence.dat file. How could I proceed?
You can rehost or transfer a license from one computer to another. You can rehost
each license up to three times within your maintenance period.
Kindly check the steps in below link - Page 35 (4.5.4. Transferring a License File to Another Computer)
Assuming your subscription is active, just copy the file to the new machine and point to it from the Quartus license settings. If it's not active, you'll have to go to the license management section of the website and renew your subscription, then download the new license.dat.