This's a known issue that our team is fixing now. 

To work around this, please go to WiiM Home App (iOS only) and select Browse tab > Amazon Alexa > Top Right corner Setting icon > Turn off the Amazon Music HD/UHD. It should enable the multiroom or stereo pairing for your Amazon Music casting. 

We'll release the fix version by Aug. 31th. together with our gapless playback fix. Please stay tuned!