I have 2 Huawei P20 Pro with Android 10 both, and a tablet Huawei Mediapad M5 with Android 9. I have the same problem with the 3 devices. This issue started some weeks ago. Before I never had a problem to connect to the Roon Core.
I have reinstalled the Rock OS on my Nuc several times. Also reinstalled the Roon remote app on my Android devices and the problem is still there. Certainly it connects on flight mode and wifi on, but is that a solution when it is been happening only since last 2 weeks more or less? Sometimes it works if I disconnect the Android device from the wifi and connect again, but after the first or second time it fails again. I have also introduce the fixed IP address of the core on the Android device clicking on Help, but it does not work.
I do not have any VPN, Firewall off, only one network, port 9003, 9100 and 9200 opnened. On the other hand, my windows 10 laptop connects instantly every time.
It is very frustrating.