Internet is required to install the eSIM, so you can install it in Mongolia and start using it once you arrive in the destination country.

Your usage days start counting from the moment the eSIM connects to the internet.

eSIM installation guide links:
Samsung: https://travelsim.mn/mn/video-esim-android
iPhone: https://travelsim.mn/mn/video-esim-iphone

The first day lasts 24 hours from the time of installation.