Yes, you should always use ‘add a device’ when you already have a system set up, and not ‘set up new system’, if I’m understanding your post. Go ahead and factory reset your latest speaker, then re-add it using the ‘add a device’ function, and it will force it into a new ‘room’, but allow you to see all speakers at the same time, and do grouping. 

Should it not be reset my old speaker and then add it to my working sydtem.

I believe that is what I said, yes. You already had a working system, presumably with some playlists, streaming companies already stored on them. The factory reset function would wipe all that data, so it’s always best to add to your older system, and reset the newest device. 

However, if you don’t mind losing any stored data, it doesn’t matter which one you choose to reset, the end result is that you have one Sonos Household/system, not two. 

Thanks. It deleted my working system, so I lost my playlists etc. hat is why I have reset my original Sonos 

So, your newer system was the more ‘correct’ system, in terms of data stored? That’s a bit unusual. Most frequently, it’s the original system which has more data/information stored, and should be used as the ‘base’ of the Sonos system. 

But, as long as you have one system now, you should be good.