@Pat_NL
Well I'm sorry I helped solve your issue since the outcome is not a positive one. A provider blacklisting a device for an unpaid account/contract usually takes quite awhile often years unless it's an obvious fraud.
It's nearly impossible to get a device removed from the blacklist unless you have made the original request or the device has been recovered and legitimately resold and the reseller/refurbisher/provider has overlooked removing it from the blacklist. (Thus contact the reseller for a refund or exchange.)
There's a very small possibility that a clerical error may have occured when the IMEI# was entered when blacklisted but that usually requires you proving you purchased the device thru a legitimate/authorized reseller or dealer.
It's highly unusual for a private seller to leave their sim card in their phone when selling it. There's a high probability that the sim card may be that of the original owner that was a victim of theft. If the sim card reveals a phone # in "About Phone" (even if the sim card is deactivated) try reaching out to see if indeed this is the case. Returning a lost or stolen device may yield a better result than the effort involved trying to get it removed from the blacklist.
If you are just stuck with the device and it's dual sim (e-sim compatible?) check the secondary IMEI# as often times only the physical sim slot or sim slot #1's IMEI# is blacklisted.
Best of luck!
