@BettyN 

Before you can set up the router's bandwidth, you must turn on the bandwidth control function, and set the outgoing and incoming speeds provided by your ISP.
- Go to the router's settings.
- In the router's settings, go to the "Bandwidth Control" tab and check the "Enable Bandwidth Control" box.
- Set the Outgoing bandwidth and the Incoming bandwidth. This is the speed that your ISP gives you. For example, if your ISP gives 100 Mbit/s for upload and download, then we need to convert these 100 Mbit/s into Kbit/s and specify it in the corresponding fields. The conversion is very simple: multiply 100 Mbit/s by 1024 Kbps = 102,400 Kbps. These data are entered.
- Click "Save", to fix the changes you have made.

Restriction of the Internet speed for individual devices

This method is more complicated. But, in the router settings, you can set the maximum speed for each device. These settings are bound to the IP address.
So, first, you need to bind the IP address given by the router to the MAC address of the device you want to limit the speed. This is to ensure that the specified device always receives the same IP address for which the bandwidth settings will be set.
How do I bind the IP address to the unit's MAC address?
To bind an IP address to a device's MAC address, you will need to go into the router's settings, under the "DHCP" / "DHCP Client List" tab. 
Here you will see a list of devices that are currently connected to the router. We need to look at and copy the MAC address of the desired device. Also, you can pay attention to the IP address that is currently assigned to the device.
If the device you want to set the bandwidth settings for is not currently connected to the router, you can see its MAC address in the settings click to read more, somewhere in the "About device" section (if it is a mobile device). 

How do I know the MAC address of my Android device?
For example, on an Android smartphone, go to Settings / About Phone / Status. The MAC address of your device is listed under MAC Address.

How to limit the Internet speed for an individual device?

Now, let's set the bandwidth settings for the device by IP address. To do this:
-Go to the "Bandwidth Control" tab. 
And to create a new rule, click on the "Add" button. A window will open in which you need to set the parameters that limit the bandwidth.
-Check the box next to "Enable".
-In the "IP range" field, enter the IP address that we have reserved for the device.
-Leave the "Port Range" field blank.
-Select "ALL" in the "Protocol" field.
-Priority. The default value is 5, you can leave it that way.
-Specify the maximum outgoing and incoming bandwidth. The minimum value, usually put the minimum allowable - 1. The maximum, I will put, for example, 5 Megabits. That is - 5 120 kilobits per second.
-I also set the Incoming bandwidth to the same value. This is the speed at which the device will receive information from the Internet. But you can specify either a higher or a lower value.
-Save the rule.

This is it, at first glance it seems difficult, but if you do not hurry and carefully follow the instructions, then you will succeed.

  @gerald_koonce

Thank you very much for the detailed answer, it seems a little complicated for me, but I will try to do everything as you said