React Native Responsive Design

I am working at startup as Front-End Developer. We are using React-Native to build mobile application. I am kind of newbee in React-Native, so I have a question regarding responsive design.

I have a mockup or sketch in AdobeXD, which uses iPhone 6 dimensions. When I launch it on iPhone 6 design appearing properly, but if I open iPhone 5 simulator it doesn`t work well.

I searched a lot of blogs and documentation, which tells using flexbox, but I don`t get it well.

So, can you tell me, should I ask a designer to make proper design for iPhone 5 too or I have to use another method of designing responsive application?

Thank you so much.