Instruction is here: How to active Developer Options Samsung Galaxy S9 The tutorial can find here: Odin flash tutorial.
Flash to your Galaxy Grand Prime Plus 2018 Combination file that you downloaded before and unpacked, by Samsung Odin select it in AP part button, mark options: Auto Reboot and F.Turn on your Galaxy Grand Prime Plus 2018 in download mode and connect to USB cable.The example we got in phone A605FXXU1AQK2 firmware then we need combination file that the beginning is the same as in our phone A605FXXU1 the rest is not important.
Combination file must be the same as firmware on the phone.