If you are looking to boot your HTC Desire 820G+ dual sim into the fastboot mode, then this is the post which is going to help you do so pretty easily and within seconds.
What is a Fastboot in HTC:
In general fastboot mode is a mode which helps you to access many functions on your android phone. With the help of the fastboot mode you. Can actually flash files with android debugging over USB with the help of a pc. It helps to repair phone when the interface fails completely or also it helps to flash recovery files, kernels etc.