Coax Digital Out -> XLR Digital Input cable?

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?