- the switch negotiates speeds etc so this means that it is not dumb, just unmanaged. My error.
No, unmanaged is what you originally said.. My lack of clarity in replying.
I meant to point out - as you are confirming - that although the switch is unmanaged it still has a lot of automatic intelligence for things like negotiation and power saving.
previously the RPi was attached to that same switch which was in turn attached to another upstream switch. It was visible on the network.
That's more interesting...
To me that suggests maybe the problem is between the trunk ('upload') port of the switch and the modem? Because the RPi-to-switch connection is apparently unchanged?
Can you provide a short word portrait of the layout/interconnect of your LAN infrastructure? Both as it was when working, and as it is now.