If your Thecus N2100 has been configured to have an IP address that is accessible from your normal network then you can access the Redboot boot loader using telnet. Telnet is on port 9000 in Redboot.

During bootup, telnet is enabled for just a few seconds. To find out when you can connect in, you can run arping in one window, while having the telnet commandline ready in another:

 arping 192.168.100.1 -w 250000

and

 telnet 192.168.100.1 9000

As soon as you see the first ARP reply from the N2100 then you can start telnet. In telnet you need to give ^C (Control-C) right away, to stop the three second timeout before it boots normally.