Marek, you do NOT need a device direct from Samsung. All you need is to have a CSC code of BTU.
So just for the future... there are two ways to change your CSC (ONLY ON UNLOCKED DEVICES THOUGH). You can change it via the phone dialpad but this will reboot your device and wipe it completely so you have to restore from backup or do a fresh build. The other way is with SAMKEY which will cost you a few £'s but will not wipe your device. Always do a full backup anyway just in case though.
Not going to go through the details as just too long but have a Google and all the info is out there for you. Use known sites like XDA to make sure it's all above board.
Changing the CSC to BTU on network unlocked devices will not trip Knox and you will retain all functionality like Sampay etc.
Hope this helps a little even though a little late for this beta
😔