To telnet into your SE208BW, follow this steps.
1. Before turning on your SE208BW connect eventual ethernet cables.
2. Turn on SE208BW and then connect your laptop, tablet or phone to it's wifi.
3. Open up your favourite telnet client (on phone or tablet, I suggest connectbot) and telnet into your SE208BW'S ip address (can usually be found by going into wifi connection info. The SE208BW's address is the first three numbers of the current device's ip address plus a 1 instead of the final number. Example: 192.168.208.3 is the ip address of my phone, 192.168.208.1 is the SE208BW's ip address.)
4. If you can't or don't want to connect to your device using wifi you can telnet into your device by;
4.1. Going into your modem's settings page usually at http://192.168.1.1
4.2. Going into your LAN settings page or in the modem status page and looking for Host-00? (? Here there should be a number often 1 i.e. Host-001). Next to Host, there should be an ip address. Telnet into that ip address with your pc connected to the same modem as the SE208BW
5. Username is root and there is the one you set for the web UI (if you didn't set a password then just press enter without entering a password).
6. When finished type exit to disconnect from telnet.