There’s a few links floating around for the Huni-Rex FB pages, one of them is a private one and one is the public one. I bought the uni from the link in my original post (Reposted here)
Assuming that’s the link you used, I glossed over some of the privacy settings on FB and nothing jumped out at me that could interfere with messaging (might be worth a deeper dive though). Could be a network issue, could be a Facebook issue 
As an alternative, @Huni-Rex posted an e-mail in this thread you may be able to contact