Currently Nomad Web connection to application servers is not possible through a passthru server.
This is required where clients connecting directly to the cloud Domino passthru server and it redirects to on-premise Domino application servers which are not exposed to the internet.
Following workaround available for Nomad for iOS and Android but it does not works for Nomad Web.
"Initial setup of the Nomad client using a passthru server connection document is not currently supported. Once the client's initial setup is complete, a passthru server connection document can be created and Nomad will use the passthru server as expected."
Limitations for iOS : https://help.hcltechsw.com/nomad/1.0/hcln_limitations.html
Limitations for Android : https://help.hcltechsw.com/nomad/1.0_android/hcln_limitations_android.html