Hi Juergen, thank you for using our forum, my name is Luis I am part of the Small business Support community.
To send the traffic to specific WAN port you could use Protocol Binding, below you will find a link with some steps to configure it, in this case you need to assign the pool to specific WAN,
Manage Protocol Binding
For SFTP you don't need to configure something special.
I hope you find this answer useful
Greetings,
Luis Arias.
Cisco Network Support Engineer.