iPhone SE Wi-fi and self assigned IP

Hi there. I have had the same phone for 10 months, the same router for 24 months, the same fibre provider (BT) forever.

my phone in the last 2-3 weeks has developed an issue where it constantly drops off the Wi-fi. Then it will show it’s on the Wi-fi in the Wi-fi screen, but not actually be connected. We have 3 other iPads newer and older which do not have this problem and a slightly older iPhone that doesn’t have this problem. Nothing, apart from the 14.2 IOS update has changed in the last few weeks. I have spoken to 2 apple support people today for a total of 1 hour. Their conclusion is “you need to speak to BT as it’s an issue with you connecting to your router, and is a Self Assigned IP Address issue”. Can anyone tell me if they’ve experienced this and how on earth this issue can only affect 1 iPhone and not all other Apple devices and how this issue can suddenly occur? Trying to get BT to fix this will be like pulling teach from a crocodile, so very much hoping someone can explain? Thank you.