I finally solved my issue.
I had correctly follow the custom item guides, but the problem seems to lie either between the client or Iteminfo.lub that cause the problem.
First, I was looking around at other server's Iteminfo.lub to compare it to mine to see what I was doing wrong, turns out that all the custom items ViewID are supposed to be 0? On contrary to guides online, I changed my ViewID to 0. That was the first fix. However, the custom item still did not appear. After that, I concluded that I'm going to re-patch my client with NEMO, nothing really different from my original patching and now. Desperately working on the fix, I login and the custom item appears and is working.
I had no idea what was really wrong, but all I did was retrace my steps and tweak a couple things, while learning the process between the client -> gtf -> server more in-depth. I hope this reply will help someone in the future when they get stuck like me.
Thank you to @OwnedGGWP for helping me over discord.
UPDATE:
So, looking into it deeper, it wasn't the client, I totally had my accname.lub / accessoryid.lub in the right folder, but wrong directory. I had it to put my files in data/luafiles514/lua files/datainfo instead of data/luafiles514/datainfo.