I have a similar problem using iPad as the control device for Roon on one of my systems, and as Paul has intimated, the problem has not always existed. My setup used to work flawlessly until around (at a guess) 6 months or so ago.
I have Roon Rock installed on an Intel NUC (8i5), and my ‘older’ iPad Air is currently running OS 12.4.6. My high(ish) speed broadband and WIFI is provided by Sky (UK), and delivered by a relatively new Sky Q router, and I also have a 2 Tb Sky Q TV box (PVR) which is also connected to the network. I have a Cisco 2960 switch attached to the router over ethernet, and the following components connected to the switch: Intel 8i5NUC; Panasonic 4k Blu-ray player; Panasonic 4k Smart TV and a sonore MicroRendu audio streamer/renderer.
My problem is similar, but not exactly the same as yours. Whenever my Sky Q TV box is connected to the router via ethernet, my iPad Air cannot find my Roon NUC. I get the same Roon Logo and message (“looking for your Roon core”) as described by Paul.
However, as soon as I disconnect the Sky Q TV box, my iPad instantly finds the Roon NUC and works perfectly. No other device on my network is affected in any way - just the iPad to Roon Rock connection.
My solution, although a bit inconvenient, is to leave the Sky Q TV box unconnected when I am listening to music and don’t need it, and to physically connect the Sky Q box when I do need it.
I use a Windows 10 PC as Roon control point for another hi-fi system elsewhere in my house, but connected to the same home network via ethernet (on a separate switch). The Roon control app on this PC has no issue finding Roon Rock on the Intel NUC irrespective of whether or not the Sky Q TV box is connected, so I guess my issue is purely wi-fi related.
I would prefer a more convenient solution!