NAT traversal is a feature of the IPSec protocol and has nothing to do with the SSTP protocol.
The SSTP protocol uses a simple TCP connection and therefore does not provide features of this type.
The problem needs to be solved on the router
You can use the AIDL interface.
However this feature is only available with a subscription or a lifetime license (excluding the Basic subscription)
Here you can download a sample app that uses the AIDL interface: VPNClientProAidlExampleV2.zip