We've now added an option for users to manually download GMS2 runtimes for cases when restricted internet access or very slow speeds will cause the automatic download attempt within the IDE to have issues.
If you feel you do need to do this manual download process, first please send us a support ticket and include a copy of your ui.log, as we may be able to address your issue - and if it is a general issue which will affect multiple customers, then of course we would prefer to fix it.
Getting the Runtimes
If you navigate to the following address: https://account.yoyogames.com/runtimes when logged into your YoYo Account you will see a list of publicly-released runtimes like the following:
If you click into a runtime you will see a list of your available runtimes based on which GMS2 products you own. This page also contains instructions on where to place the zips when they are downloaded.
You will always need the Core Runtime and also whichever platform(s) you wish to develop for.
Still An Issue?
If this still fails to install your runtime, please report a bug and attach your ui.log file, which can be found in the %programdata%\GameMakerStudio2\ (Windows) and /Users/Shared/GameMakerStudio2/ (Mac) folder.