Help me - I am stupid

Hi.

I need your help.

I managed to install Tailscale on my devices I want to connect (NAS, Android Phone, Chromebook).

The devices are shown as “connected” in the dashboard.

If I try to connect to a service from one device to another, I get a timeout - not matter what service I try to use (i.e. Plex Server, NAS Web Frontend, etc.).

What am I doing wrong?

Is there anything special I missed?

Do I have to open specific ports on my router?
BTW: I use a FritzBox with a dualstack IP connection to my ISP.

Thank you for any hint that leads me in the right direction.