Hey there,
There is no offline installer for Diablo IV. However you are able to download the game on one network and transfer the Data folder to another PC on a different network to avoid data caps.
You could start the Install on your home PC to get it to create the folder structure, then pause the download once it gets to the point of downloading the bulk of the data (XX.XX GB remaining @ XX.XX MB/s).
Then fully install the game on the other PC with no data cap. Copy the entire Diablo IV\Data folder over to your home PC and replace the mostly empty Data folder with the one from the full install. Restart the Battle.net app and it will scan everything to make sure the game is up to date. At that point you should be good to go and only need to download new patches.
Hope that info helps!