The 210 has an IP address ... how would it work with Roon?
The issue is whether an iPhone can identify the 210’s IP Address, to communicate via Wi-Fi, not Roon.
That is not an issue. The iphone would be on the same network as the 210 and would be using the internal network's management tools to see the 210's IP address.
Typically most devices will receive an IP Address from a DHCP server. The machine that runs DHCP typically has a list of all of the allocated IP addresses (and names).
If you have mconnect, you can test the principle by turning off your bluetooth on your phone and try and stream a file to a 210.
Cheers
Tom