I'm a complete networking idiot and I've fallen at the first hurdle.
I have an R610 here, just hooked it all up and powered it on. Checked the iDRAC settings (made sure it was using the shared NIC, as I don't have switch space to go dedicated).
The IP address stated in the iDRAC settings is the default static 192.168.0.120. The IP address set in the R610 UEFI is set to 'Dynamic - DHCP'.
When I check the connected devices at my router page, I can only see the servers mac address, connected at 192.168.0.50. If I change the server to a static IP, it remains at .50. If I set the iDRAC to dynamic, I still can't see it connecting to my router, and the server remains on .50.
Whatever I do, I can't access the iDRAC web interface via 192.168.0.120 nor 192.168.0.50.
Added information from second account of OP
The subnet is the same. 255.255.255.0.
I am OP, though I posted the question as guest. For some reason it wouldn't let me comment thereafter.
Some more info:
If I set both the iDRAC to DHCP and the R610 (via lifecycle controller) to iDRAC, neither show their assigned IPs in the interface. If I check the connected devices to my router, it still only shows the first NIC port connected, at 192.168.0.50.
So DHCP will assign the mainboard an IP, but not iDRAC.