Google Ad Manager only supports asynchronous rendering. So, you don’t have to do anything to enable it. However, you have an option to switch b/w SRA (Single request architecture) mode and multi-request mode. In SRA, all the ad requests for the page view are sent at one go from the header, whereas in multi-request mode, each ad request is sent separately to Google Ad Manager. If you’re running roadblocks, it is advisable to use SRA mode.