What tools / guide did you use to accomplish this (building the ROM AND adding your certificate) ?
But for beginners there are some pretty nice LineageOs build guides floating around:
For specific pieces of hardware: https://wiki.lineageos.org/build_guides.html
Generic instructions: https://forum.xda-developers.com/chef-central/android/how-to...
As far as changing the certs, I know offhand to do it with a couple random linux distro's but i'm not 100% sure for android, you might just try searching the repo for the default certs then looking at how they are built into the image and tweaking that.