If iPhone Disconnects from WiFi whenever it is locked or goes in sleep mode, you will have a hard time making calls using WiFi Network.
WiFi Disconnects When iPhone is Locked or in Sleep Mode
While some users have reported the issue of WiFi Disconnecting from WiFi Network after an iOS update, this problem can occur at any other time due to Network related issues and other reasons.
You should be able to fix this problem using troubleshooting steps as provided below.
1. Renew DHCP Lease
Obtain a New IP Address for your iPhone by renewing the DHCP lease and this should help in fixing the problem.
Go to Settings > WiFi > select your WiFi Network Name.
On the next screen, scroll down and tap on Renew Lease option.
On the confirmation pop-up, tap on Renew Lease to confirm.
2. Turn OFF WiFi Assist
The problem might be due to the WiFi Assist feature switching your device from WiFi to Cellular Network.
Go to Settings > Cellular > disable Wi-Fi Assist by moving the toggle to OFF position.
3. Disable VPN
If you are using VPN, disable the VPN service on your iPhone and see if this helps in fixing the issue.
Go to Settings > General > VPN > move the toggle next to Status to OFF position.
4. Forget WiFi Network and Rejoin
Note down your WiFi Password and follow the steps below to Forget WiFi Network and rejoin the Network.
Go to Settings > WiFi > select your WiFi network.
On the next screen, tap on Forget This Network option.
On the confirmation pop-up, tap on Forget to confirm. After 60 seconds, rejoin WiFi Network by entering the Password for your WiFi Network.
5. Disable WiFi Networking Service
Some users have reported fixing this issue by disabling WiFi Networking service.
Go to Settings > Privacy > Location Services > scroll down on the next screen and select System Services option.
On the next screen, move the toggle next to Networking & Wireless entry to OFF position.
6. Restart Modem/Router
In above steps did not help, the problem might be due to a glitch in Modem/Router.
Switch OFF the Power Supply to Modem/Router – Use the ON/OFF button (if available) or unplug the power cable from wall socket.
Wait for 60 seconds and switch ON the Power Supply to Modem/Router.
7. Reset Network Settings
Note down your WiFi Network Password and follow the steps below to Reset Network Settings on your iPhone to original or Factory Default settings.
Go to Settings > General > Reset > On the next screen, select Reset Network Settings option.
On the confirmation pop-up, tap on Reset Network Settings again confirm.
After this, connect your iPhone to WiFi Network again and see if the problem of iPhone not connecting to WiFi is gone.
8. Switch to Google DNS
Switching to Google DNS can help if the problem is due to the DNS server of your Internet Service Provider being busy or going through problems.
To use Google DNS, you can refer to this guide: How to Use Google DNS On iPhone and iPad