I am trying to make wax material using a gradient and an emission (in the screenshot on the left). It looks good, but if you look at the mesh from behind, when it is in a dark corner, it looks weird. I also tested material without emission (in the right), it looks like cheap plastic.
I know that emissive material always lit. So any advice on how to make wax more realistic is useful to me.
You might want to try switching the lighting model for the wax part of the candle to Subsurface Shading. This will simulate how light is scattered through natural materials such as wax and skin.
Take a look at Subsurface Shading Model | Unreal Engine Documentation for a little more information.
Those look mostly like wax. On the right, without emissive, that is how lots of candles look without light transmitting through it, or if it’s not lit. They simply have that appearance of a cheap plastic. To accentuate the waxy texture, try softening the lighting on it by decreasing metallic, roughness, and specular. Then put a softer texture sample, or modify a texture sample to be softer looking in terms of a candle surface, in the normal slot of the main node. And definitely use a different model / blend mode, particularly the one @smelnick97 suggested. About that dark corner weird look, try making it two-sided to dampen the weird effect because it sounds like the small amount of light in the dark corner when viewing behind it is oversampling or doing something to enhance the lit part of it. Two-sided can thicken the material and make the lighting as it is closer to a solid, translucent object.