Settings>About Phone>Software Information>Service Provider Software Information
Most of these three digit code must be set to BTU with a few exceptions which show your carrier etc.
This tells the Samsung servers that you phone is eligible to enrol in the beta. You can change these codes if you are OK to get a little technical and be OK with the possibility to have to rebuild your phones or restore a full backup.
There are two ways to change the CSC. One is via the Phone Dialpad with a sequence of #* and digits, this will automatically reboot and WIPE your phone. The other is via a Windows program called SAMKEY, you have to pay for a 'token' to do this which costs a few $'s, you have the option here to not wipe your device. However your device MUST be Network Unlocked to do this, it will not work on Network Locked devices. One thing to keep in mind is that going to a CSC of BTU should then tell the Samsung Servers that your device is eligible for the beta you may lose functionality like WiFi Calling as I believe that for Vodafone and EE you need their specific firmware with a CSC of VOD for it to work, well this used to be the case. Also certain things like Google Pay will not work on Beta releases due to security reasons. All info on how to do this is available on reputable websites like XDA, make sure to do a FULL BACKUP before you try anything and follow instructions to the letter. Please remember that you make any changes like this at your own risk. Whilst this is a widely used process, especially via the Phone Dialpad there is always a small risk that something may go wrong and you may brick your device. Extremely unlikely but just wanted to paint that out.
Hope this helps a little. I cannot give detailed instructions as each device is different and the process varies albeit only slightly, not to mention I would be here all night typing 😉.
If you do anything then post back and let people know as it may help others. Best of luck!