How To Stop Annoying "Cellular Network Not Found" Popup Prompt?
The popup you're referring to exists as to alert the device user that the cellular service as become unavailable. If you wish to stick to a specific network, there's nothing you can do.

Options for Mobile Networks are located at:

Settings → Wireless &amp; networks → Mobile Networks


And there's no option to prevent the system from warning you about the lost of network connectivity.

From the above location, under "Network operators", you have an option "Select Automatically", that will allow your device to select another network without disturbing you.

But by your question contents, you don't want this.