iPhone 15 Pro Max Wi-Fi drops after a few minutes

My wife recently purchased a refurbished iPhone 15pro Max from Apple. It connects fine to our Sky Max router but drops the connection after a couple of minutes browsing. A Network WiFi reset or using the Forget Network option is a workaround but this has to be done every few minutes whilst browsing.

Measures already taken:

Restarted router (same effect as network reset or disable and enable WiFi)

Set security level on router to WAP2

Fixed private WiFi address

Please advise if this is a common problem