Ben owns both the Vistron and Tevion Reciva radios and encountered problems listening to RealAudio streams through his Zyxel P660HW-T1 router with v3.40 firmware. After a bit of research, he discovered the solution.
Ben has kindly provided step by step instructions:
When you have the above screen visible, scroll down in Service for REAL-AUDIO(TCP:7070). Highlight it, and click the Add button to the right.
Click your way back using the Apply button at the bottom of each page.
RealAudio, such as on BBC Listen Again will now play perfectly.