How is your TRS cable wired?
L/R outputs from the Solo are line-level, and inputs on the NU are stereo, so you just need a basic "AUX cable" that converts 2xRCA to 3.5mm.

Oh right, those are 6.3mm jacks on the back of the Solo. I wasn't looking closely at the photos and thought they were RCA. It should work with one or both channels, as long as the wiring is correct. First thing I'd check would be the adapter plug, as you're converting 6.3mm TS to 3.5mm TRS, so there's room for error there (e.g. many 6.3mm adapters are TRS on both sides).

So I tried Mono to Stereo Y Cable and still does not work on either. Does not pick up mic at all but the Scarlett does.