Hi
In Totalmix, open the toolbox of Hardware Input 1/2, there you can find the switch „48V“.
Or wasn’t that your question?

There are lots of informations in the manual and also you can find helpful videos (by RME) about TotalMix.
Anyway some important facts:
With the TotalMix app you do all the mixing for the physical outputs of your interface (headphones, analog line out, digital ADAT/SPDIF/AES outputs).
Every output (mostly set to stereo pairs) can have its individual mix.
As sources for such a mix you can select hardware inputs (top row) - just like on a mixing desk - and also the software playback channels (mid row).
To do so, you set TotalMix to 'submix' mode (you find that in the blue area on the right side of the window). Then select your output (probably headphones) by clicking on this channel strip and open the faders of the channels you want to hear.
In the case of hardware inputs you do a so called direct monitoring: the signal flows directly from the input to the output. Although it passes the AD-DA converters, it stays in the unit only. The USB connection as well as the computer and the audio software on it are not involved so far.
At the same time, the input signals (for example your mic) of course also are sent over USB to your computer / DAW. This signals are not affected by what you are doing in TotalMix (of course except things like activating phantom power 48V and adjusting the input gain).
A DAW and some other applications using the audio hardware normally have a software monitoring function which - if enabled - makes the received signal going back to the interface.
In TotalMix such a signal would be appear in a channel of the 'software playback' row.
What your DAW (or other software) shows as 'output' is always directly related to the 'software playback' channels in TotalMix, NOT to 'hardware outputs'.
That means, your mic audio signal can be heard twice in your headphones or speakers. Latency free coming directly from the hardware input and with some latency over USB back from the computer.
It depends on your needs, what signal you want to hear.

Are you sure you have the fader of input 1 open when the headphone output is selected?

Input gain is fully closed !
Open the gain and signal will come.