Ubuntu 21.04 - bluetooth 5.0 dongle does not work and says there's no firmware

The firmware for rtl8761b is missing in your Linux distribution if you see these errors:

Direct firmware load for rtl_bt/rtl8761b_fw.bin failed with error -2
firmware file rtl_bt/rtl8761b_fw.bin not found

Execute these commands to install the missing firmware:

cd /tmp
# Fetch rtl8761b_config and rtl8761b_fw from https://github.com/Realtek-OpenSource/android_hardware_realtek
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
mv rtl8761b_config /lib/firmware/rtl_bt/rtl8761b_config.bin
mv rtl8761b_fw /lib/firmware/rtl_bt/rtl8761b_fw.bin
sudo modprobe btusb
sudo systemctl start bluetooth.service
hciconfig -a # will show that the bluetooth-device is up

If you are looking for a more comfortable solution, here's a similar, script-based solution that automatically installs missing bluetooth-firmware, including rtl8761b: https://unix.stackexchange.com/a/643707/88252