I can see a few people have asked the same question but not for my model of wifi modem. My Chromecast "can't be found" and Google tells me to turn off my AP isolation. I've gone through every tab in the device but can't find AP isolation. I have a Huawei HG630 Home Gateway
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We've checked the Manual for the Huawei HG630, but was not able to see the AP isolation on it.
You may need to contact the Chromecast support to check further the needed configuration aside from disabling the AP isolation.
Here's the manual for the HG630 (https://www.192-168-1-1-ip.co/manuals/8600.pdf) for reference.