This
should work...
A headphone signal is about "line level" except it's
capable of driving lower impedance headphones and the headphone output almost always has a volume control where sometimes a line-output does not.
If the output is stereo
you need to "split" the left & right into inputs 1 & 2. You'll probably need a combination of cable adapters...
"Mono"
TS Adapters (like a guitar plug) might be "safer than a TRS plug because a TRS plug might make a balanced connection* (the headphone output is unbalanced) and it's just easier to get the correct connections. Then you'd need a cable like
this.
This probably isn't necessary but for "deep troubleshooting" you might try a headphone Y-splitter so you can plug in headphones at the same time to see if somehow connecting the interface is killing the signal.
*Stereo into a balanced connection results in left minus right subtraction and that would foul-up the signal in "strange ways". With regular stereo music it makes a "center channel vocal" remover, which removes everything in the center (everything identical in both channels) including the bass. If the output is actually dual-mono.... Both channels identical but with a stereo plug so you get sound in both ears... Of course subtraction gives you silence (or near silence).