Fiber Optic Internet...best way to connect to system

The simplest things to try at wildly varying prices:

The Etherregen (basically a switch) is advertised as being useful in between the gateway and the node but they are unavailable as of this month while they develop a new version with more readily available parts.

The Gateway has an external power supply that could be replaced by an 12V LPS with at least 5.5amp current delivery.

Purchase an audiophile switch to go between the Gateway and Node - there are many that are advertised.

Less Simple but not necessarily higher cost than a power supply for the Gateway or an audiophile switch:

Buying 2 switches with SFP ports to connect together with fiber to isolate potential noise on the network. Further, you could add a power supply to each router. Check the Trendnet TEG-S51SFP.

You could get the switches and fiber cable for under $200 and a power supply for each switch for between $110 and $$$$. I’ve had very positive results with a low priced power supply on the switch that connects directly to my streamer.

While one may ask, "the Bluesound is $550 so why spend as much or more in other components?" - I would argue the Bluesound sounds good and whatever the cost of ones streamer, the network equipment feeding it may benefit from enhancements.

Then there’s always the ethernet cable itself. My favorite is actually fairly low priced and available on Amazon. It’s a 22awg CAT8 cable that is the most neutral and detailed cable I’ve owned. If one is looking to warm the sound, it won’t do that, it seems neutral to me which is something the Bluesound on its own can use with its hint of pleasant warmth. Check it out if you’re interested: https://www.amazon.com/gp/product/B08DP4JW56/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&th=1

OP:  There's nothing you can do  to get around the cable modem.  Leave it alone.

My best advice is that you use iFi power wall warts for any Ethernet devices near your audio chain.  For instance, I use an Ethernet switch to provide a cable to my audio streamer, AVR, TV and DVD player.  Switching the power supply there may keep your analog devices from picking up noise from the AC.

You can insert an optical connection (can be done for about $80) between the router and your Node that will shunt away the inherent noise, then you can get a better DAC if you’re currently using the Node’s internal DAC.