Firstly, make sure you don’t have MR “extensions” installed. This is the app that links Cubase to the MR816 and srcews everything up if you’re using the RME Totalmix. Once you’ve un-installed MR extensions, go into the MR editor and route the first pair of adat inputs (1&2) (and nothing else) to the analog outs 1&2 (first tab on the right of the MR editor). Do the same for 3&4, 5&6 etc. Now go down to the first adat tab and route analog ins 1&2 (panned hard left and right) ONLY. Do the same for adat outs 3&4, 5&6 etc. What you’re trying to achieve is, via the MR editor, sending one channel of analog out to one channel of adat and visa versa.
See my other post on this subject: