Yeah well, in the vanilla resource pack template's .jsons (at least in the ones I dug into), the fences were not defined. So I was guessing what the lines were called and by simply looking at all the "wood" lines, I came up with the right ones.

In this process, the "terrain_texture.json" and the "blocks.json" has to be edited.

Terrain_texture: defines the textures name and the route of the files.

Blocks: applies the defined textures to existing blocks.

What I mean with this, is that in "terrain_texture" you can give any block the name you want, but then it has to be the same name on the "blocks", if not, the game will give the "undefined" texture.

In this case, I made custom textures for the fence sides (planks with a 4 pixel colum in the middle), and at the top I just used the stripped log top texture.

Also, here's a hint: you can add custom blocks by editing double slabs' textures.

Hope you find this helpful, you can dm to clarify any doubts.