The DAC has to "see" the mR boot up, meaning it has to be on when the mR boots up. It sometimes takes more than one attempt.
Is your mR up to date? If not, update and try again.
If you really can't get it to work, put in a request to Sonore. They can look at your setup online and get it working.
Also, try another USB cable and/or the other USB input on the Mytek just to see.