Hi there, I had same problem with my Chromecast 2nd Gen suddenly not working with the 5G SAGEMCOM FAST 5866T modem....you can't disable the AP Isolation on this modem. But you can activate one of the Guest Networks through the Online Dashboard (username/password on bottom of modem). The Guest Network section is at the bottom of the dashboard homepage. If you turn on ONLY a 2ghz Guest network, then reset your Chromecast, and then connect the chromecast to this Guest network, it will connect to the 2ghz band only, and not the 5G as default. Note, the Guest network has a different password which is displayed on the dashboard page. If you connect your phone/tablet to the same Guest network as chromecast, the chromecast icons and functionality will appear in your apps! It's not ideal to have to switch to the Guest network, but it's meant I can use my chromecast again...hurray! ... View more