I think your question is not necessarily within the category ("brain …") you placed it in 😉 … I think it is a good general question: What to do in case of forgotten passwords?
You did forget your admin and/or user password for the web interface, right? Yes, these can be "reset" via sending a new configuration to the F454. But what if one forgets the so called OPEN password (old or new HMAC-based authentication)? Without that password, one won't be able to download any new configuration — unless one uses the unfortunate "IP range" thingy to circumvent authentication, which is generally a very very bad idea.
So let's say one has forgotten the OPEN password and does not have any "IP range" exception enabled? What can we do?
For firmware version v1.x there is one way into the system: The by-default password-less root user. The firmware is running a getty process on the system's first UART, which is wired to the front-panel UART connector (below to mini-USB). So login is possible there, including the possibility to edit the config. files manually (removing the password).
v2.x firmware finally got a root password, so this backdoor is closed.