Mediclaim CignaTTK

Hello sir, I had purchased mediclaim policy for my Dad who is Sr. Citizen in the year November 2017. Insurance company had done medical check-up as well. at the time of disease declaration he was diagnosed as Diabetic since 2011. In the year 2015 got Jaundice and Liver Cirosis which we failed to inform the insurance company accidentally. in the year December 2019 my dad had to go Galbladder operation. After putting the claim insurance company said rejected claim because at the time of enrollment policy disease was not declared by policy holder. can you suggest way out to do anything.