I created a custom wall, which works fine in every aspect except for the fact that walls don't connect to each other.

Here is how I initialize the wall:

CHERT_BRICK_WALL = new WallBlock(Block.Properties.from(CHERT_BRICKS)).setRegistryName("chert_brick_wall");

All the JSON files are copied from vanilla:

models:
chert_brick_wall_post.json
chert_brick_wall_side.json
chert_brick_wall_inventory.json

 

Where could the error be?

Also, how do I make creative tabs?

You need to add them to the 

minecraft::walls

tag.

Question remains about the Creative Tabs though!

Easily solved with this code for creating the Creative Tab in my Main Mod file:

public static final ItemGroup ITEM_GROUP = new ItemGroup(MODID) {
    @OnlyIn(Dist.CLIENT)
    public ItemStack createIcon() {
        return new ItemStack(ItemRegistry.berylGem);
    }
};

and using the group method on the properties builder