Allo USBridge USB connection can do 192, but Digital Coax doesn't

I am running Roon (on Apple iMac) through an Allo USBridge connected to a Benchmark HGC2 DAC. When connected through USB, I can listen to 192K FLAC files but when I switch to Coax Digital, I can’t listen to those files. Benchmark owners manual says Coax can play up to 192, so it shouldn’t be the Benchmark. The sound is slightly better through Digital Coax than USB but not at the cost of not being able to listen to my high Rez files
does anyone have any ideas about what I should do?

The DAC has a number of LEDs indicating input selection, word length and sample rate. In particular, the input selection indicates any error with the audio signal via four flashing rates.

What LEDs are lit when you change to the coax digital input?
Does the coax input LED flash?

Edit - I’m assuming the digital cable is rated at 75ohm.

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Huh? Digital Coax input is RCA cable.

Just repeating the advice from Benchmark.

“Caution: Use 75-Ohm coaxial cables for digital audio connections D3 and D4. Digital interfaces require the use of matched impedances. Do not use 50-Ohm coaxial cables, twisted pair cables, or any non-coaxial cables for digital audio. The digital inputs may not function, or may be unreliable if the incorrect cable is used”

Yeah, probably just being cautious.

I think that most RCA type coaxial are all 75-Ohm, therefore your assumption was correct.

The only cable I’ve noticed that one could get 50 vs. 75 Ohm is with cable that have BNC connectors.

Pretty sure it is 75-Ohm cable, is there any way to test or check?
When I run a high res file through USB, the USB light on the Benchmark lights up (solid) and the 24 bit word length lights up as do the 48K and 4X (IE:192k) but when I switch to Coax, the D3 light (Coax) blinks blue and the 24 bit word light and the 48 and 4X lights blink very lightly, you can barely see them. Hopefully the enclosed photo will show what I am seeing

From the user manual, page 11

Input Error Codes
The input indicators flash when errors are present on the selected digital input.

There is a table of associated messages which I cannot copy.
So please refer to the above link to the user manual.