Incremental capabilities aside for a moment, which automatic routing function you should use in general depends on the nature of the signals being routed. NanoRoute (ie, "globalDetailRoute") is typically used for routing signal and clock nets. "sroute" is typically used for routing power/ground connections so it doesn't seem well-suited to your purpose (I'm inferring this from the PortA/PortB names you mentioned rather than VDD/VSS).
Automatic routers in the tool rely on logical connections being in place which instruct the router as to which connections should be made. For signal nets, these connections are determined from the input Verilog netlist primarily, which can be altered in-session with ECO commands like "addInst", "addNet", and "attachTerm". You could verify these connections by clicking the terminals and querying their net and flight lines.
Once these connections are logically in place, you can proceed with incrementally routing them with "globalDetailRoute". It would probably be good to "setNanoRouteMode -routeWithEco true" to inform the tool that routing is present and the tool should incrementally refine the routing.
Hope this helps,