Is the Digi One SP unit online? The unit will need to be on-line and accessible by the host in order for the tty device to be created.
If the unit is on-line, please confirm it is reachable by the RealPort host on the proper port by issuing the following commands from the Linux root prompt:
# ping 10.1.2.5
If you can successfully ping, test the port:
# telnet 10.1.2.5 771
If an error is seen, you will want to ensure port 771 is open on any corporate firewalls.