There is an excellent office switch you can buy cheaply called the Cisco 2960. Despite huge production this business/office switch was still hundreds of dollars when new back in the day as it is built very well with an excellent internal power supply. Bought Used it is only a few tens of dollars these days. These switches have a great reputation in the audio world as the sound quality is comparable with the best audio switches. And it has an optical output which will connect directly to the OpticalRendu > via usb to dac.
Modem/Router > via Ethernet to Cisco 2960 switch > via optical to OpticalRendu
Also you would connect the Cisco 2960 via Ethernet to your Server.
Like all optical connections you need a little SFP module to be inserted into the OpticalRendu optical port and the Cisco optical port. I suspect that the Optical Rendu comes with one so you will just need to buy a compatible one for the Cisco. They only cost a few dollars.