Any Mobile application if it has to get connected to the internal corporate servers which are protected by firewalls cannot directly be accesse unless the ports for these servers are exposed to the external network directly or through a reverse proxy , It is exposed else VPN is the direct way.
1. In your case , the most easiest without changing your landscape , First connect to VPN over Mobile to your corporate network first and then try to login in. It should work.
2. Else, you can hosts a Reverse proxy server in the DMZ layer and expose to the external network. In the reverse proxy server, you can map the internal Mobile BI server details which again can as well help you.
3. Tird is , you directly expose your Mobile BI server in the DMZ layer so that it is accessible to the external network directly , In this case , your IT has to as well manager the security since your Mobile BI server is directly exposed to external network.