You can directly limit their mobile internet speed through the router’s backend, but I feel there’s no need to do so. Unless your internet speed is really pitifully low.
To limit download/upload bandwidth, you can use Quality of Service (QoS) settings on your router
If you router doesn’t support that, go and buy a new one
Do you have the access to the backend of the router? If so, login in,maybe you can find the list of the devices that have connected.Detect the device that you would like to limit and see if you can limit their speed.
Yes I have access to the backend. But DLINK router doesn’t have QOS service. That’s why I don’t have control over that and that’s why I am finding some third party apps.