Complaint regarding used Poco C51 from Snapmint

I place a order of phone (Poco C51) threw Snapmint – EMI on 9th December 2023 and I received the phone by 11th December. I used this phone for few days and started getting switch off on 20th December 2023. So on 21st December I went threw Xiaomi Service Center at (116, Gala Empire, Drive In Rd, opp. Tv Tower, Nilmani Society, Thaltej, Ahmedabad, Gujarat 380059) they told me that this phone already activated on 23rd November 2023 and currently having an problem with software so I asked them is there any way that I can got replacement. So they said not you have to contact Snapmint. Then they have update the software and give it to me on  22nd December 2023 but having an same problem. So I was there again on 23rd december in morning and again they did something with phone so its work for 1 day and facing the same problem again in afternoon of 24th December 2023. So I due to holiday I go there on 26th December 2023 again I strictly tell them that I want replacement so they told me that is not possible because I have used phone and there policy says we can replace the phone with in 7 days but how can I replace it if I dont have any problem in 7 days and started getting an problem after 10 days.

Please check it and let me know that how you gone help me with this.