I didn't change anything apart from uploading my own png and changing the font file to it. Not sure how to actually load it ingame though.
Viewing post in Hakuen Studio Bitmap Font for RPG Maker MV MZ comments
Your font file must have the same amount- and the same order- of characters listed on the plugin parameter "all characters".
So if you created a new PNG to be a bitmap font, it needs to follow that rule above.
You can take the default one that is on the sample project as an example of how to create your PNG(BITMAP) font file.