Hi Stephen,
I’m no network guru and never will be, but probably I’m able to narrow the root cause a little bit.
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First idea: What happens, when you connect your ST directly to the router?
-> if Roon works, there could be a problem with the settings in your switch, then go to idea number 2
-> If Roon doesn’t work,… Well, no clue then. -
Second idea: I’ve read some threads regarding managed switches and flow control (look here, for example), probably there is a faulty setting in the switch.
Good luck 