Why is this group closed? I think there would be nothing wrong if unregistered users could view posts and it would be better, because your community would develop faster, because more people who are not followers contributed to this community. A friend of mine decided to make a similar Instagram community dedicated to his new car. He gained followers from 1394TA and started posting information and life hacks about his new Tesla. I think this is very convenient, because the posts are better sorted and look more detailed due to Instagram photos. Personally, I would like such content much more and I think many people would too.
"Groups" (Facebook or otherwise) do not magically stay open. There has to be a moderator for them and "community values of the host" need to be maintained. It's THEIR orientations, as they own and host the situation. Might have become the target of spammers? Perhaps in need of a new moderator? Or perhaps the "traffic" decreased too much? Many possibilities.
Perhaps that one group disbanded and was re-invented under another name? I'm not on Facebook, so I don't know.