How to root my phone if all rooting tools are not supported?

I have a Samsung Galaxy J2 Ace, running Android 5.1.1. I want to root it now, and I have installed its respective system firmware from Frija. Initially, I followed David Bombal's tutorial on how to root Android (he used Magisk), but later found out that it is only supported for Android 6+, and so it is not applicable. I have tried Kingroot, Kingoroot, SuperSU, Magisk, Ding Dong Root, etc., and none support the device. I decided that if I unlock the bootloader and then upload a custom ROM using Odin, the phone might get rooted. Just asking if this technique will work. (I am a Windows/Linux user and don't know much about rooting, just had a useless phone) If not, then how?