Hey there, the only ports open/mapped are the ones noted in the sandbox instructions. As the gRPC is not open inbound to the sandbox, this would not work even if you enabled it on the device. In such a case, to get this to work using one of the reserved sandbox and tunnel this over the VPN.
Hope this helps!
SSH Port: 8181
NETCONF Port: 10000
NXAPI Ports: 80 (http) & 443 (HTTPS)
RESTCONF Port : 443 (HTTPS)
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