For freelance clients, they often want to see work you've completed that is similar to the work they want done. Unfortunately, when you come from a traditional job, you often can't use most of your work in your portfolio. The work around for this is to create public side projects and make them available for clients to view. They don't need to be huge, they just need to be on point for the related tech. This is especially important for back-end development because it's not as visually oriented.
Don't discount your full time job experience. It's just as valuable and can be used to strengthen your CV. If you are moving from a regular job to freelance you can try and find freelance jobs that have a strong match to tech you worked with. As long as you can show that you can work with those technologies and you can talk in depth about them, the client will pick up on your capabilities.
Tldr; Build a public portfolio with personal projects then lean on your professionl experience to sell yourself.
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