How to go about getting my seniority back after being let go while on sick leave for cancer

How do I go bout getting my seniority back After my company so called outsourced my position after 11months into my job! I was diagnosed with Largyneal cancer and my doctors put me on sick leave my company was okay with it at first this was march 7 2019! May 1,2019 they asked me to come in to work so I did and they let me go and took my insurance away to! After I beat my cancer 8 weeks later they found out I beat my cancer and called me and offered my job back same position! I was collecting unemployment from them but I decided to go back to work but I got everything back insurance pay rate even a raise but they said I had to lose my seniority and start over! How do I go about getting my seniority back