Air Play not working

Hello Community,

i am so frustrated because of the following Problem:

I bought an apple tv 3 (rev 1469) and airplay worked properly with this apple tv in the previous environment.

But in my network the apple tv simply does not show up on my iphone SE (latest ios11) or on my iphone 4 (ios 9)

WLAN Router is a Fritzbox 7390, with no firewall or restrictions like that.

Air Play is definitely ENABLED, with no security features.

The apple tv is successfully connected to the wifi network, and i can use youtube etc with optimal performance, no network delay, the signal strenght is shown with 4/5 points . I even can remotely control the apple tv with my iphone se.

I resetted the iphone network settings, i resetted the apple tv twice, i unplugged/restarted the devices (appletv & iphone) serveral times.

I did a hard reset on the iphone.

I tried 2,4ghz & 5ghz wifi networks.

I took a 10€ access point, resetted him and set up a small mini network, attached the apple tv with ethernet, he gets a proper dhcp address from the new router. iphone is successfully connected via wifi as well, apple tv remote app works without problems but it is simply not shown in the airplay list. It says "apple tv is being searched" or in german "apple tv wir gesucht..."

What is wrong with that apple TV?

I tried absolutely everything from the official apple support links, so please dont refer to them.

I hope some guys out there can help me, thanks in advance! Greetings