4G-AC53U and Google Chromecast

[kvartbok]

Hi,

I have the 4G AC53U router and I'm trying to get the Google Chromecast to work together with it. Until recently I could use the 1.Gen CC together with the router, but it stopped working. So I tried to reset the CC, the setup went fine but I can't connect from the Google Home app, it just says "Connection ended" right after I try to connect. And the CC is not visible for other apps, like youtube, netflix etc.
So I bought the 3.Gen CC, and I have the same issue there, but there I also get an error icon on my TV screen. I'm reposting the icon from Google Support Forums.

Airtime fariness is disabled
AP's are not isolated
I can ping both the CC's from within the network without any loss of packets.
The router has the lastes firmware.
Both CC's has latest available firmware.

Google Support suggested to setup an WiFi network from my phone and connect another phone and try to isolate the problem, but I don't have a second phone or tablet at the moment. So I'm trying here first.

I hope anyone can help, thank you in advance.