I have had a TS-451 for a couple years now and I decided to graduate to bigger and better NAS units to do most of my work. The TS-451 is now dedicated as a surveillance system using QVR Pro. It works great.
Anyhow, to completely reset to factory default, press and hold the reset button on the back for about 10 seconds. You’ll hear a beep around 3 to 4 seconds - keep holding it longer. The first beep just resets your network and admin settings to default. Holding it the longer time resets to factory default.
Once you do this, you should then find the unit on your network via Qfinder Pro. Then you can access it and start the web setup.
#1 thing to do - update to the latest firmware. Some units out there run older firmware that is not as secure, etc.
#2 thing to do - change the default admin password and then disable the default admin account. Newer firmware versions force you to do this basically.
Also - just understand that on your TS-451, the CPU usage will be very high at times - 100%. It’s like that for me a lot. It’s somewhat underpowered.
Also, I would recommend upgrading to the max amount of RAM - I think it’s 8 GB.