Lewis spent part of last season on-loan with Accrington Stanley.
He told the London Evening Standard: “It wasn't an easy decision to make but I felt like at this stage of my career that it was something I needed to do to gain experience.
“I had a good offer from Union. It was a good opportunity to play first-team football and start my journey again.
“They really wanted me and had a good plan for me. It hasn't happened for me but I believe it will. It was difficult to leave Chelsea but in football, you sometimes have to take the risk.
“I joined a great team that finished high up the table and it is like a family. But it was difficult for me, especially at first, being away from home. It was difficult for me to settle. I had a few things going on both on and off the field that made it hard to settle in."
Lewis adds: "When you see people you've played with doing well then you also want to do well. We all have different pathways. I don't look at them and think they're doing well and why am I not. We all have a different path so I just look at myself, keep grounded and work for it."