At 5 meters, i would be curious to know what will you be actually testing: the cable, the transmitter or the receiver. All toslink cables attenuate the same? I would be surprised if yes.
That's part of the issue.
Here, those devices seem to be mere repeaters.
They detect a signal, don't care about its format, and repeat it.
The longer (or badder) the cable, the worst the result will be.
I wonder
1. If repeating the signal will help for longer distance
2. If there is a point they won't work anymore, anyway
3. If the RME will be able to work whatever the cable length or if, as it is likely, it will just stop working at a given attenuation (where no DAC would work anyway)
There are a lot of unknown here.