SSH reverse tunneling on Android topic

How to get access to Android (3G connection, no external IP) from the Internet?
If I set up a reverse SSH tunnel to port 22 of Android device:
ssh -f -NT -R 1234:localhost:22 user@shell_server.com
it works fine and I have access to the device through user@shell_server.com:1234
But if I set up the reverse tunnel to another port:
ssh -f -NT -R 4321:localhost:8080 user@shell_server.com
it doesn't work:(

Probably this requires an additional tweak on the SSH server

/etc/ssh/sshd_config

#Match User anoncvs
# X11Forwarding no
# AllowTcpForwarding no
# ForceCommand cvs server

But this file is read only. How to change it? Is it a right way? Maybe there is an app to set up a reverse ssh tunnels?