WiFi connection problems?

Hi all!

iv been having a problem for a while with my WiFi connections, it was happening on my iPhone 7 and now my iPhone 8 which I have had since November, it’s been happening since I got this phone but it’s getting very frustrating.

My wifi disconnects when I lock my phone, and does not reconnect when I unlock the phone I have to go into settings manually and reconnect again...

does anyone know know how to fix this?