Suggestions for Wi-Fi issues
Below is a User Tip about Wi-Fi issues. Suggest starting from the top and working down. Maybe one of them will help.
1) Restart you device. Restart your iPhone, iPad, or iPod touch - Apple Support
2) Reset Network Settings: Settings > General > Reset > Reset Network Settings. Rejoin the Network again.
3) Reboot Router/Modem: Unplug from power for 2 minutes and plug again. Update Router Firmware (check manufacturer’s support website for a newer FW). Also try different bands (2.4 GHz and 5 GHz) and different bandwidths (20 MHz recommended for the 2.4 band). Channels 1, 6 or 11 are recommended for the 2.4 band.
4) Change to Google's DNS: Settings > Wi-Fi > Click on the Network, Delete all numbers under DNS and enter 8.8.8.8 or alternatively 8.8.4.4
5) Disable device prioritization on the router if this feature is available. Also disable any VPN apps and test Wi-Fi again.
6) Determine if other Wi-Fi devices on network are working well (other iOS devices, Macs, PCs).
7) Try device at another Network, i.e., neighbors’, public coffee house, etc.
8) Backup and Restore the Device using iTunes. Try restoring as New first and testing it. If ok try restoring the backup (the backup could be corrupted).
https://support.apple.com/en-us/HT201252
9) Go to an Apple Store for hardware evaluation. The Wi-Fi chip or the antenna could be defective.
Thanks to Meg St._Clair for suggesting that I write this tip.
This user tip was generated from the following discussion: since I installed the 9.3.1 IOS wifi signal is weak or lost.