Am sure by now you have read about the differences between 2.4GHz and 5GHz Wireless. If you are connected to a 5GHz Wi-Fi network, switch the Wi-Fi antennae to look for Wi-Fi networks in that particular band. This might improve Wi-Fi performance. You can switch to a 2.5GHz Wi-Fi band for better performance.

Recently we had talked about the history of Android and mentioned Google Android 5.1 Lollipop. This is a version I have liked very much and not moving to Marshmallow anytime soon, the iteration brings several welcome features such as the lovely icons, animations and the multitasking menu were completely redone with Google’s Material Design approach,support for multiple SIM cards, the Android lock screen, device protection and high definition voice on compatible phones from the days of the Android 4.0 Ice cream, it also got the inbuilt WiFi signal Boost Settings but the Wi-Fi connection seems to be creating problems for users after the Android 5.1 update.

Following are a few potential fixes which could help you deal with the Wi-Fi issue.
 

Restart your Android device and Wi-Fi router  

Rebooting the Android device might do some good in resolving the Wi-Fi issue. You can also restart your Wi-Fi router for this.

Changing Wi-Fi bands

Am sure by now you have read about the differences between 2.4GHz and 5GHz Wireless. If you are connected to a 5GHz Wi-Fi network, switch the Wi-Fi antennae to look for Wi-Fi networks in that particular band. This might improve Wi-Fi performance. You can switch to a 2.5GHz Wi-Fi band for better performance. 

To change the Wi-Fi frequency band on your Android device, go to 'settings', 'Wi-Fi' then tap on the three dots on the top right corner of the screen a drop down menu will appear, select 'Advanced' then tap 'Wi-Fi frequency band' and select the frequency that matches the frequency used the router you want to connect to.

Remove/re-add Wi-Fi networks

You may be needed to remove the Wi-Fi network from the list of networks on your Android device so that is can re-connect afresh. 

To "forget' a known Wi-Fi network, go to 'settings', 'Wi-Fi' and select from the list of networks the one you want to forget then tap on the 'forget' button. Afterwards, switch off your Wi-Fi and switch it back after 20-30 seconds. You might be able to connect to the Wi-Fi network on your Android phone.

Change DNS servers

Changing the DNS (Domain Name System) might also fix the Wi-Fi issue if the above methods have not helped. Follow this steps, got to 'Settings', 'Wi-Fi' and switch on the Wi-Fi.If it is a known network which you have once used, you may have to 'forget' it and re-connect again. When you get the Password window popup, click on 'Advanced options' and select 'Static' from the IP settings drop down menu. Then scroll down and enter the IP address of the DNS server you would like to use in DNS1 and DNS2 box. Now enter the WI-Fi password and connect it.

Factory reset

This option must be the last ever if you have no otherwise on getting your Wi-Fi to work. Factor reset will wipe everything in your Android device so that you can deal with your device like a new device. There are two methods to go about Factory reset that you can do. First is done by going to 'Settings', then to 'Privacy' then select 'Factory reset' option. Second is the Hard reset method where you have to switch off your device then switch on my pressing both the power button and the volume down or up buttons together at the same time for like 4 seconds. If Factory reset won't resolve your issue the it could be a hardware-related issue. You might have to take your device to the service centre to get it fixed.