I have a Roipeee (Raspi4 with Hifiberry Digi+ 1.7 Build 571), it can connect to the Roon-Server (QnapTS-253Be 1.7 Build 667), at least I see it as Roon Bridge under about. Unfortunately this roipeee does not appear as a playback device under Audio.
A while ago I could connect, but now I have reinstalled the Roon Server and the Roon Bridge.
Is there any change in behavior if you temporarily try to host the Core on your PC? As in does this issue only happen when the QNAP is the Core? To try the other PC as a Core:
Open Roon on the other PC you wish to try as the Core
Roon Settings -> General
Disconnect
On the “Choose your Core” screen, press “Use this PC”
If asked to Unauthorize, you can go ahead and do so. You are limited to one active Roon Core at a time but you are free to switch between them as often as you’d like.
Verify if the same behavior occurs on the different PC