Got the M5 and loved the smooth setup/app features. It's plugged into my ISP rental router combo as the modem. I had it in the recommended operation mode of "Wi-Fi Router" (Fast Roaming was disabled). After linking my Chromecasts and Nest Minis to the new network, I was unable to cast to them from YouTube or YouTube Music (using a Google Pixel 5a with Android 12). I could cast from YouTube Music to the Chromecast Ultra, but not to the Nest Minis (the speakers would ping and the YT Music app would show I was connected, but music would only play off my phone). I was unable to cast from the YouTube app on my phone to either Chromecast—when attempting to do so, it would go so far as to open the YouTube client within the Chromecast, but not actually play the media I had selected. So, attempting to cast a video kind of prompted the Chromecasts to "start up" YouTube, but the M5 appeared to prevent casting media. Vexingly, I was able to cast media from Google Photos without issue. After LOTS of Googling (I am not hardware tech-savvy), I couldn't find this issue replicated. So on a whim, I switched the M5 operation mode to "Access Point" and am now able to fast from YouTube and YouTube Music normally.

The reason I would call this "sorta" solved is because running the M5 as an access point makes me lose out on security features within the app, as well as about 1/3 of my speed (down to 29mbps download from 50mbps—upload is unaffected).

Questions: can I run the M5 in "Wi-Fi Router" mode and still be able to cast media? Was there a setting that I could have manipulated to make this possible? Alternatively, would using my ISP's rental router as the modem create this issue? If so, is there another way to hook up the M5 and preserve all the features? 

Thanks!