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Be very careful with that device. The PIX line is end-of-life/end-of-support/end-of-patch with known vulnerabilities with public exploits. Particularly the 501/506 since they only support up to 6.3. You would be better served with a 2000 era PC running pfSense or something similar.
@user1172468 The 515e is the bottom grade, but most modern class, of PIXes. It'll take the newest PIXOS but is still abandoned technology. So better, but still a bit scary. That device does use standard desktop RAM, I can't remember which exactly, so it is easy to upgrade out of support.
If you have the serial cable to match you can connect it. 9600 N/8/1 is the default connection. Boot it to see what OS you are running. Then you can run the factory reset procedure detailed in the other posts. At that point you may have a startup wizard available when you reboot.