Well, I tried different ways but as I could not make MR816 be selectable in another way than Master CLOCK, finally I’m using this configuration:
Audio: M AUDIO firewire to my Mac. RME and MR816 through ADAT OUT to M AUDIO ports ADAT IN A and B respectively.
Sync: MR816 Word Clock OUT to M AUDIO Word Clock In, and M AUDIO Word Clock OUT to RME Word Clock In (and RME has a button to terminate signal).
MR816 master Clock, M AUDIO receiving CLOCK and automatically send it through CLOCK OUT.
It is working with no problem, so I’m happy in this way. I think MR816 has a kind of bug or something else with Clock connections because as seen in this post and others, I’m not the first to have this kind of issue.
Thanks @shanabit for your replies!