If i understand properly, you want to create an ssh tunnel from your machine behind the ISP and expose its service through a port on the SSH server.
If thats the case your command line appears correct. I do something similar.
You have enabled the Gatewayports in the sshd config so all that is good, if you didnt do that you could only bind the port to the loopback address 127.0.0.1 on the ssh server.
Quick note, I had to reboot my ssh server for the gatewayport option to start working. Restarting the sshd service should be enough but it didnt work for me.
I see that you arent specifying a port for the ssh server which means you are using port 22, your ISP may be tempering with it in some way.
One other thing, whatever service you want to access on the server behind the ISP, make sure its port is binded to the 127.0.0.1 interface cause that's were your pointing your tunnel. Also, check that your service is accepting connections from that interface and double check the port its running on.
@barlop -R 127.0.0.1:12345:localhost:12345 would be wrong, the service would only be accessible to someone connecting on the loopback adapter.
But reading barlop's post I realized something, keep in mind the way you set your command the port that is being opened on the ssh server will be bound to the same IP running your SSH server. Which means the service is being exposed to the internet, maybe its what you want but if the ppl who need the service connect from the LAN then you need to bind this port to the LAN ip of the ssh server. ALSO are you SURE that ssh server you are connecting to isnt behind a firewall which is forwarding port 22 to the serveir internally? if thats the case it means you are asking the ssh server to open a port on an ip that belongs to the firewall, which is impossible obviously.
In that senario you would need to open a port on the ssh server's own lan ip, then the local machines could connect directly there, if you wanted to expose the service to the internet then you need to forward a port from the firewall to the port you opened on the ssh server.
Just trying to cover everything, its hard to get a clear picture of whats going on without more info on the infrastructure.
Awww the pleasures of networking.