That’s because in the world of docker, each container gets their own internal docker network with a different subnet and therefore are isolated from each other.
For apps to communicate with each other they need to share a common network or you have to use internal dns.
Vote for this feature request.
In the comments of that feature a community member shared a script he wrote that can be used to get inter app communication to work.
Another option would be to install portainer from the apps catalogue, manually create a “share” network and manually add it to the apps you want to use.
The same could be done via cli with plain docker commands
docker network create -d bridge networkname
docker network connect networkname containername