Fitbit - Product not fit for purpose

I bought a fitbit charge * in November. In February it developed * faults after their ** day refund period. The activity tracker did not work, and the screen display developed lines through it. After some time I was issued with a replacement.. After * months the second device developed the same problems. I have a heart condition and need this heart monitor for my health. I now cannot trust fitbit as both devices are not fit for purpose in the same way. I have asked for a refund. Fitbit are refusing a refund and told me to contact Amazon and to follow their policy for refund as I purchased it through them. I did so and they asked me to get a service denial letter from Fitbit so they can proceed. Fitbit are refusing to give this. So I am now stuck with a $*** device that is not fit for purpose.

In fact they have said and I quote:"We already did all the troubleshooting steps to fix your Fitbit Charge * but to no avail" It is still broken. It is still under warranty. It is the second device to develop the same problem. This is why I require a refund.