But I cannot find my ROON core (Qnap NAS) from PC/Ipad side. They are connecting the same router already. Just curious what happened with my ROON server.
Roon Server on your QNAP is not Roon OS Core. Roon OS is either Roon Nucleus or ROCK (Roon Optimized Core Kit), a specialised Llinux based appliance. The screen shot you show is only for Roon OS devices (quite misleading if you ask me…)
You should look for “Select a core” or similar when you start Roon remote on your PC or iPad