Try turning your radio off and on again (from the mains) and then restart UNDOK.
Try restarting your phone.
For iOS14 and above, if UNDOK has access to location ensure that Precise Location is enabled.
Try "Clear app data" under UNDOK settings (Android only). If your radio is still not discovered after this try uninstalling UNDOK and then install again. These steps often solve discovery issues.
You also need to ensure that the 'Keep network connected' setting on your radio is set to yes. This will be under your system settings on your radio.
Try turning your Wi-Fi router off and on again.
If you have VPN software running on your phone or local network please ensure that the VPN is configured:
If you have IGMP snooping (and/or IGMP proxy) enabled on your Wi-Fi router try disabling this. UNDOK (and devices supporting multi-room) rely on multicast traffic which can be blocked if IGMP snooping is enabled.