Claim Declined from Care Health Insurance as Pre-Existing Disease not declared

• I have taken Care Health Insurance for my Mother in Mar 2020. While taking Health Insurance she did not have BP, Thyroid so we did not declare as there are no pre existing diseases.
• In Feb 2023, like every year we(Me, Mother & Father) went for Full Body Checkup and it was then found that 3 of the 4 walls blocked/clotted for my Mom and Doc said that she has got a silent heart attack as your mother does not have Sugar she did not feel it.
• As per the feedback we went to Care Hospitals and got Angiogram done to confirm that she has been living on 5% of pumping from 1 valve which is also 90% blocked.
• After checking with other friends and family we chose Star Hospital, Hyderabad as they are expertise in Cardiology and have best doctors across india.
• So Mom was admitted and operated for a By-Pass surgery where they took a nerve from her thigh and connected it to one of the Valves for blood pumping in heart and also put Pacemaker which will support the heart when there are irregular heart beats.
• She got operated in April 2023 and through medical tests done in hospital which they conduct before she undergoes By-Pass surgery they have learnt that she has BP or thyroid which we did not declared at the time of taking insurance and claim was rejected, i provided insurance department with a proof of the full body checkup she has undergone in feb this year and that is when we got to know that she has BO & THYROID and it is not a preexisting disease, same documents were submitted as a proof to the insurance company as a response for declining the claim, Couple of points here i.e

1. Firstly, We go to know that Mom has BP and thyroid when we took the full body check up in Feb 2023, which I submitted to insure the proofs which we got from the MD which is the 1st consultation. I responded with the documents that Doc gave us showing that on Feb 17,2023 only through full body check up she got to know that she has BP & Thyroid.
2. Secondly, I was going through Policy details of Care Health Insurance where it said after 3 years of taking the insurance, insured will be eligible for diagnosing any type of health issues as this is part 3 years after taking insurance. 

As insurance was declined, I paid up with my credit card and got mom discharged.

I went to the Care Health Insurance agency and showed the same proof that in Mar 2020 when we took the policy she did not have any Pre-EXISTING DISEASES and we got to know feb 2023 but they had no answer and said we can't accept the claim.

Please suggest how to go about it as i had to pay up 17 lakhs even though i have 35 lakhs coverage from care health insurance which i could not avail.