Running away from repairs

Got a phone July 2021. Went to get it repaired because after a few months the battery lasts only about 5 hours. Store tells me they can't help me because its an online order and I don't have a proof of purchase. Eventually got hold of the guy from online store that sold me the phone. He emailed me the delivery note with invoice. That was last week Thursday. Today, which is the following Friday, the store contacted me to tell me the booking was rejected and my phone will not be repaired. This is useless. How do you not know when I got the phone from you? Its on contract, I make payments every month