Download and install RLT8761B chipset firmware:
wget https://raw.githubusercontent.com/Realtek-OpenSource/android_hardware_realtek/rtk1395/bt/rtkbt/Firmware/BT/rtl8761b_config
wget https://raw.githubusercontent.com/Realtek-OpenSource/android_hardware_realtek/rtk1395/bt/rtkbt/Firmware/BT/rtl8761b_fw
sudo cp rtl8761b_config /usr/lib/firmware/rtl_bt/rtl8761b_config.bin
sudo cp rtl8761b_fw /usr/lib/firmware/rtl_bt/rtl8761b_fw.bin
Unplug the USB dongle and plug again.
To fix problems with the bluetooth headset microphone not being detected: https://atish3604.medium.com/solved-bluetooth-headset-mic-not-working-detected-in-ubuntu-20-04-86a5236444d0
Thank you! On my 5.13 kernel I had to use: