Coax Digital Out -> XLR Digital Input cable?

No that looks like a microphone cable or analog you need digital.

https://www.sweetwater.com/store/detail/PDRXF10--pro-co-10-foot-pdrxf-10-premium-digital-cable-10-fo...

Just try an adapter first and let your ears be the judge.
Or go right to a Hosa.
Bi directional pro SPDIF to AES /EBU conversion.

You need conversion device. S/Pdif and AES3 (balanced) have different input and output levels and also different characteristic impedance.  One is single ended while the other is differential.

In the past I used Canare transformer, but it is limited to 6MHz. It is ok for redbook CD (16/44.1) or even 96kHz S/Pdif, but it will not be enough for 192kHz (12.3Mbit/s) that requires at least 12MHz.

https://www.bhphotovideo.com/c/product/367655-REG/Canare_BCJXPTRB_BCJ_XP_TRB_75_Ohm_to.html/overview...

You could also try using Toslink optical since both the Node 2i and Boulder support this.

Toslink direct is usually better than converting coax.

Thank you.

I broke the Toslink flap door already, so I don’t want to use Toslink. 
(I hate this door... too fragile.)

Btw, I thought that Coax is better than Toslink (even better than Glass Toslink cable.)
https://www.whathifi.com/advice/coaxial-vs-optical-vs-hdmi-which-is-the-best-audio-connection-to-use

It turns out I need the converter to use AES input.
I’ll order this converter to use a RCA cable (75ohm).

I'm curious why you aren't using the built in streaming support in the Boulder instead of using the Node 2i?

Long story short, Apple doesn’t allow Boulder to use Airplay (though it’s capable.) And Boulder doesn’t support Idagio Streaming that I use.

To use Boulder streaming support, I need both Qobuz and Roon with Roon Core. I do not want any of these.

For most of music I listen to, the SQ is CD. The rest of quality is slightly better than CD. A little bit of upsampling doesn’t give me any noticeable difference. So, music source is CD quality, and the easiest way to stream music is using Node 2i streamer that has both Airplay and Idagio Streaming.

Got it. Hope you can get it to work the way you want. 

If you are interested, for a bit more money you could consider a Denafrips Gaia. This is a digital to digital converter and will accept SPDIF in and has AES outputs. It has optically isolated inputs and reclocks the data with an oven controlled oscillator.

I just got one of these a couple weeks ago and I'm very impressed by what it has done in my system. I'm using USB in and i2s out. 

https://www.denafrips.com/gaia

Coax cable provides DIGITAL signals.  XLR cable is ANALOG.  

Never the two shall meet :-)

Coax cable provides DIGITAL signals. XLR cable is ANALOG.  

Never the two shall meet :-)

You should google "AES/EBU"

XLR is a connector type, while AES/EBU (now known as AES3) is the name of digital standard (IEC 60958 type I) that uses 110ohm cable and XLR connector.

Oh no.  Digital and analog confusion again.  Look, the signal is electromagnetic, it does not know if it is digital or analog.  Both types will travel at the same speed and react to impedance the same way.  So let's just close the case on getting a digital or analog cable.  Silly topic.  Now, you can DESIGN a cable for digital or audio applications, but basically you are designing in the parameters of impedance, etc. 

You are splitting a signal, single ended to balanced.  That can work OK. But that will halve your impedance.  Any type of signal has to be matched to that at the source or load.  So you should start with knowing what your source and load impedance is. So if you mismatch the source and load and the signal is analog, it is more forgiving.  Of course you have trashed the signal.  If it is digital all hell brakes loose and if you have a good receiver maybe you can recover and not lose too much data. 

I doubt a straight cable is your answer.  Probably an adapter, but never designed any so can't say. 

You should try this

https://www.bhphotovideo.com/c/product/880882-REG/Neutrik_NADITBNC_MX_Digital_Impedance_Transformer....

with that you can put the adaptor into the back of your 866 and a standard coxial cable from node to back of that adaptor.