When we receive Android updates, the android system prevents instant download with metered connection, suggesting the use of WiFi. Large android updates of over 500mb size can only be downloaded within a specific time frame on GSM network, usually the very early morning hours, however, there are tips to immediately access and install OTA updates in the absence of WiFi.

Using wifi can involve putting the SIM with data in another smart phone with hotspot option and tethering back to the device needing the update or just connecting to a wireless infrastructure and downloading directly.
On metered connections, scenarios like what is painted below surfaces and you do not even see any download button on the update. To reveal the download button, follow the steps below

Switch off your mobile data or WiFi.

Go to your device "settings".

Tap on "date and time".

Turn off all automatic time setting options, in my case, "automatic date and time" and "automatic time zone".

Since these options are off, you can now manually set the time. Choose a time within your Android update range. If i had 3am to 5am. I'll Set 3:05am for example.

Now, check back your system update and you will find the download button. Make sure you have enough GSM data for the download.

http://pccona..com.ng/2017/10/how-to-download-large-android-update.html?m=1

What do you mean by GSM Network. Are you referring to downloading a 500Mb file with 2G network speed or what exactly.