raspi:
openssh installieren incl sshd
systemctl status sshd
evtl. /etc/ssh/sshd_config anpassen:
   port 23456 ?
   X11Forwarding no
   AllowTcpForwarding no
   PermitTTY no ?
   PermitRootLogin prohibit-password !
   LogLevel VERBOSE | DEBUG3 (for debug use, errors) 

( Permit root to login via ssh only with key-based authentication 
    see: https://unix.stackexchange.com/questions/99307/permit-root-to-login-via-ssh-only-with-key-based-authentication
)

systemctl restart sshd
systemctl status ssh


( On some Linux systems, SSH authentication messages appear in /var/log/secure 
  or only in the systemd-âjournald log, which can be viewed with journalctl -u -r -f ssh. 
)

***   router / firewall (FRitzBox)
/ Internet / Freigaben / Portfreigaben:
   neues Gerät hinzufßgen, TCP, Port 23456, Freigabe aktivieren

***   run on smartphone:
#ssh gerd@109.250.10.250 -p 23456 
ssh gerd@192.168.178.127 -p 23456