In the current era of the communication technology, the great majority of the smartphones of today, especially the Android 7.1.1 Nougat, are applying the 4G technologies for the cellular network communication. In the counties or the regions which lack of latest state of the art technological advancement, the 3G cellular networks is also under usage or even the 2G cellular networks are also being applied. The provision of the 4G or 3G technologies depends upon the carrier provider as well. But, one thing is crystal clear; if you want the best performance from your Android 7.1.1 Nougat, what you need is the fastest 4G communication service.
So, you must understand that if you are in the region where 3G service and you have moved to the region where the 4G service is available, it is a good idea to convert your smartphone service, immediately, to 4G. In most of the smartphones, this service is changed, automatically, but, if your smartphone does not have the facility of automatic conversion, you may do it, manually, with only few commands. On the other hand, if your smartphone is using the higher carrier service network and the same is not available in your current region of smartphone usage, your smartphone may not work, satisfactorily. So, you must convert the service network of your smartphone to the most appropriate one to work with it, effectively.
If you are an expert user, you may just follow the command line and change the service network. But, if you are novel to the Android 7.1.1 Nougat, you may need to see the illustrated Tutorial.
Command Line: Home > Settings – More > Wireless & Network – Cellular networks > Mobile Network Settings – Preferred Network Types > Preferred Network Type – 4G / 3G / 2G.
Tutorial
1. Tap the icon of the ‘Settings’ app, in the bottom right corner of the Home screen of your Android 7.1.1 Nougat device. This icon can be moved from here to any other place upon the Home screen but, currently, I have placed it here. 
It will open up the ‘Settings’ app screen.
2. At the ‘Settings’ app screen, tap the ‘More ….’ in the ‘Wireless & networks’ section to open up the ‘Wireless & networks’ settings interface of your Android smartphone.
3.Tap the ‘Cellular networks’ in the ‘Wireless & networks’ section, to open up ‘Mobile network settings’ screen.
4. At the ‘Mobile networking settings’, see the default status of the ‘Referred network type’ is ‘3G’. If you want to change it, tap it to open up a prompt, ‘Preferred network type’.
4. At the ‘Preferred network type’ the current status is 3G. If you want to change the cellular network settings, you may tap the desired one.
Note: As my network is currently, supporting maximum ‘3G’, ‘4G’ is not available upon the prompt. But, if your region and the carrier is supporting the ‘4G’ network, it will appear here as well and you may opt this change, accordingly.
That’s all!