This issue is due to WiFi connection between iOS device and router/AP

There are a number of things this could be, please try in the order below:

1. Background apps

• Close down any background apps or services that may be using the devices power
  

2. Low Power mode activated

• Disable low power mode as this shall cause issues
  
• If the iOS device power is less than 10% suggest recharge battery and retry   
  

3. Restart iOS device

• The most popular fix is to do a complete reboot of your iOS device, as this wipes any cache and restarts all the services
  

4. Disable Bluetooth settings:

• Open Settings
  
• Select Bluetooth
  
• Toggle Bluetooth to “Off”
  

5.Check the below basic router settings and restart the router to clear cache memory 

• SSID: Set to “Any Unique Name”
  
• Hidden Network: Disabled
  
• MAC Address Restriction: Disabled
  
• Security: WPA2 Personal (AES)
  
• 2.4GHz Mode: Set to 802.11b/g/n
  
• 2.4GHz Channel Width: Set to 20 MHz
  
• 5GHz Mode: Set to 802.11a/n
  
• 5GHZ Channel Width: 802.11n: Set to 20 MHz and 40 MHz / 802.11ac: Set to 20 MHz, 40 MHz, and 80 MHz
  
• Channel: Set to Auto
  
• DHCP: Set to Enabled, if it’s the only DHCP server on your network
  
• NAT: Set to Enabled, if it’s the only router providing NAT services on your network
  
• WMM: Set to OFF
  

Now and again Apple release updates that can cause issues, please follow the above common fixes