connect to server with IP address running sql-express instance through sql-management studio

I have my VPS server in a remote location with ip address 172.16.11.2 (for example) having two instances of sql-server express running.

one instance is Microsoft SQL Server 2008 R2 (SP2) where I connect from my local PC using remote desktop with the following credentials:

 servername : 172.16.11.2 authentication : sql - server authentication userid: xx password:xx

the other instance is:

Microsoft SQL Server 2008 R2 (SP2) with advanced services. To connect to this machine I remote desktop to the server and type in

 servername : machinename\sql-express authentication : sql - server authentication userid: xx password:xx

now using remote desktop from my local machine how will I connect to this instance of sql-server.

I tried

 172.16.11.2\sqlexpress 

but this failed. I also enabled sharedmemory/namedpipes/tcp-ip but still I am not able to connect. When I go to the machine where sql-express is installed and then type machinename\sqlexpress I am able to connect the sql-express instance, but from my local machine using sql-management studio if i type in ipaddress\sqlexpress I am not able to connect. It gives me network related issue error.