I know nothing about Ableton, and you said precious little about your setup, so all I can do is offer a little generic advice. You probably need to open MR Editor to set up the I/O configuration to get what you want, then match up the connections you specify in Ableton to the setup you’ve chosen. Sorry, but it will involve learning enough tech through reading the manuals (both for Ableton and the MR816) to understand what you need to do.
In Cubase, this is normally done through VST Connections, but the MR816 defaults to having any ports set up as external I/O ports turned off, and there’s no way to change it in Cubase. So we have to open MR editor and set it up the way we want, then save it as the default configuration in MR Editor. It will then work OK in Cubase.