For Roon being able to play to an audio device, this device must be connected and available… So, did you make sure your DAC is connected to the Allo Bridge and switched on?
I moved your message to the Support section of the forum.
So you are using a Allo DigiOne HAT, not a Allo Bridge Signature as you mentioned in your original post?
What does that mean exactly? You mentioned that Roon in its About page can see the Pi with RoPieee… so that would mean that it is definitely available on your network.
Can you connect to RoPieee with a browser, using the IP of the Pi4 or the name you have given that RoPieee instance? Is your DigiOne HAT correctly configured in RoPieee?
Edit:
By default you should be able to connect to the RoPieee instance with http://ropieee or http://ropieee.local