@QuarryHunslet - are you using the UPnP/DLNA Bridge plugin? If so do these devices work fully with it e.g. respond properly to play/pause, gapless etc?I'm not sure, but this DLNA Casting feature is intended for a WiiM device to be able to cast to other DLNA enabled speakers, such as Sonos, Bose speakers, and more.
My RIVA Festival, and Arena Speakers support DLNA up to 24 bit 192 kHz, but have never worked properly with any of the betas so far.
They also don't work with the released version of DLNA Out on the Ultra.
They do however work properly if I cast from Lyrion Media Server on my Raspberry Pi, and support the full 24 bit 192 kHz as expected.