connect to server with IP address running sql-express instance through sql-management studio
Ensure that the secondary instance (in this case SQL Express) has a TCP port set.


Under SQL Server Configuration Manager, go to SQL Server Network Configuration, then Protocols for [INSTANCENAME]. 
Right click TCP/IP and under the IP Addresses tab make sure the field "TCP Port" has a value (for example 1533).


Then try to connect to the instance with the newly assigned port:

172.16.11.2\sqlexpress,1533


Voila!