You probably need to add extensions to the ring group to get the extensions to ring. A phone in DND will show that on a display.
You can do this all from keyset programming.

Defaulting the system and starting from scratch sure.Is it easy? Nobody can tell you that as we don't know your configuration.Could be easy, could be harder. I am guessing you have little/no experience with Samsung programming? How are you going to do that?

Without knowledge and passwords you are going to be in over your head.

IT tool is old and depending on the software may not work properly. OSDM is what you want.