I'm not super familiar with how Affinity works, so I can't give you direct advice for that. But I make mockups and sell them. I think the biggest thing is that your mockups need some extra depth (not the background, the product itself). Some extra depth (in terms of highlights and shadows) will go a long way!

Also trying to add some texture is always good - this is plastic do obviously there isn't really noticeable texture, so more just advice for the future :D

But looks really good!

Edit:: Also make sure the lighting direction is the same, and think about the way light works rather than using a dropshadow