I have enabled USB debugging as well as OEM unlocking developer option in my Infinix x510 which is running Android 5.1.1. I found that to reboot into fastboot mode, I have to turn off the phone, then hold down both Volume Up and Power button at the same time.
In my case, when I follow the said key combination, the Infinix logo comes up, goes away then reappears, keeps doing so until I let go of the keys. It then follows the standard booting procedure and boots into Android OS.
How can I get into fastboot mode? I want to get into fastboot mode, flash TWRP, then via TWRP install the latest Android OTA update manually.