Configure (travel) router as "media bridge"

I've been struggling to configure my travel router (tried both the Opal and the Berryl AX) as a "media bridge" (like ASUS calls it) to bring wireless connectivity to ethernet-only devices (in this case, the Holo Red audio streamer).

I don't understand why this is so hard to get to work...

My previous setup consisted of a Nest router (Wifi 5) acting as the main router for the whole house and a separate ASUS RT-AC68U in "Media Bridge" mode - connected to the Nest SSID - to bring internet and network connectivity to my streamer. This works great overall, but I would like to get something smaller, hence the GL.iNet travel router.

However, I am unable to get a reliable connection to the streamer when using either router in Repeater/Extender mode, which I believe is the closest to "Media Bridge".
I use Roon to play music to the Holo Red and it's really hit or miss whether it will show up as an endpoint and when it does, 90% of the time the playback just doesn't start and the tracks just skip.

The Asus router does not require any configuration once it's in "Media Bridge" mode. I don't believe it acts as an extender in this mode either, just a WiFi to ethernet bridge really.

How should I configure my travel router to achieve a similar behavior?
I don't need firewall, I don't need VPN or any other advanced features, simply a reliable WiFi to Ethernet bridge.

If that matters: the main router is a Flint on 192.168.8.1. I set the travel router to 192.168.10.1, since it otherwise conflicts with the main router.

Thanks in advance!

Actually, setting the Berryl AX to WDS mode fixed the issue.
The Holo Red device is now getting an IP address on my main router’s subnet (192.168.8.x) instead of the travel router’s (192.168.10.x).
I can access the Berryl AX’s configuration UI at the IP the Flint gives it, too.

Now the question is: why? What’s so different with WDS compared to extender? Why would the Holo Red be visible but having so many issues in extender mode, while everything seems smooth in WDS?

WDS is "real bridge" which works in layer 2.