Trouble with Updates

Hello! I have been a bit confused about how to update my Macbook. I am currently having MacOS High Sierra and live in India which is important later on. I decided to update the software to the latest MacOs i.e Catalina. But once i downloaded the package from App store, i again have to install it using the MacOs installer. I am not able to as I get limited data of net per day. Hence when I try to update to Catalina, it takes a lot of time and then I have to cancel it again cause of the net issues. This restarts the update bar.

What should I do? Guide me with steps on how to download the latest OS update.

P.s Since i didn't upgrade to MacOS Mojave, I can't pause the Catalina update through System Preferences.

Then I thought I should first update to Mojave and then be able to update to Catalina without any issues of the net data finishing. But is it the same issue with Mojave as with Catalina? Will I face the same problem above once I download the Mojave Installer.

I never had to worry about the network once I download the installer in the previous versions of Mac.

Pleas help me. I am in a Fix. Also bear in mind that I cant update in one single go. I'll have to do the process daily to update it.