Eventually, after two weeks, Samsung Customer Service replaced my Phone with a new one.i would like to Describe my Experience with the new Phone.
Now I have New Galaxy A51; my Phone has Android 10Q with Build NumberA515FXXU1ASKJ. My previous GALAXY A51 had updated to A515FXXU1ASL6 that had a lot of problems with Bluetooth and WiFi.Because of my experience, I stopped updating the software on the new Phone. The Phone works perfectly without any problem. Bluetooth works without stuttering and keeps connection always.WiFi works perfectly, and the Phone never freezes or lagging. But my memory usage is unusual; in the last 24 hours, my average usage of memory is 3.5GB (60% of 5.8 GB available memory). You can see in this picture; six applications get around 2.4GB of memory in one day.
I think GALAXY A51 problems occur in new Build Numbers (A515FXXU1ASL6 / A515FXXU1ATA4). Thus, please do not update your Phone until Samsung solves problems in new versions. Please, anybody, have Bluetooth issues; please write your 'Build Number' in comments.
For finding your 'Build Number', you should go to the address below:
Setting > About Phone > Software Information > Build Number