wifi auto diconnect when lockscreen

Hello.

Have you tried to do something about it? Do you use VPN? Have you tried to disable it?

First step is to forget this network and then rejoin the network. By forgetting the network, you will erase all the settings related to the particular Wi-Fi network that may be turning off Wi-Fi when the iPhone is locked or sleeping.

If the issue isn't resolved by this, it is not necessary that the problem is associated with your iPhone. Sometimes, Wi-Fi router can also cause problems like Wi-Fi disconnects when iPhone is locked. You can try updating the router firmware if it is outdated. As every router manufacturer has a different process of updating the firmware, visit the official website of your router manufacturer to get the detailed information about the process.

Also try to turn off the Wi-Fi Networking option under Settings > Privacy > System Services.

Come back with the results.