Thank you for your fast reply.
As I only have a limited data volume (3GB/month), I affraid, that a complete VPN connection needs to much data volume.
Currently I have installed a reverse SSH connection only between 2 Raspberry (one per each network) with very limited data volume demand. The problem is, if the remote raspberry freeze, the connection to the remote network is lost.
Therefore I prefer a reverse ssh connection between the remot RUT240 and the local Router or local raspberry as ssh client.
Is there any possiblity to install a ssh server on the RUT240? It must initiate the ssh connection to the local router/local raspberry.