I should add here, that there is also a thing called the “GSMA blacklist” that most worldwide carriers use.
https://www.gsma.com/security/resources/imei-blacklisting
After checking several online free checkers using the stolen device’s IMEI #, it shows as “Blacklisted” on them all.
EDIT: The only thing that stops this list being effective, is thief has an insider at a carrier, who can presumably take the device off the blacklist again. 