Please turn off then turn on Roon Ready using Lumin app. If that does not work, power cycle the Lumin.
If it still does not work, power off all network devices including Roon core, Lumin, iPad. Power cycle the Wi-Fi router. Wait for 5 minutes, then power up Roon core. Wait some more, then power everything back up.
Please describe your network topology and model of router etc.
oh…
TP-Link Deco M9 Plus router(s)
wired connection to a TP-Link Jetstream T2500G-10TS switch
fiber connection to a TP-Link MC220L Gigabit SFP media converter (TP-Link TL-SM311LS SFPs at both ends)
wired connection to a Cisco SG100D-08 v2 switch connecting:
late 2012, 2.3GHz i7 quad-core Mac Mini (Roon Core), Qnap HS-251+ NAS and Lumin U1 MIni
yes: IP addresses for both Mac Mini and Lumin are reserved on the router (and both were fine)