I recommend XMPP than Signal, especially for Chinese people. I think XMPP's main features are anyone can select from many servers and it have easy to use End-to-End Encryption implementation.

"Is Jabber accessible from China?" - reddit
libreddit. (webproxy frontend for reddit) URLs: clearnet, Tor

I'm in China and can access many XMPP servers, e.g. xmpp.jp, swissjabber.ch, chinwag.im, yax.im, disroot.org, member.fsf.org.

From what I understand about China's GFW, and I am American so no direct experience, it'll work at the beginning if you use TLS. Maybe not STARTTLS?

However, I do know that all VPNs will eventually stop working, forcing you to switch IP or protocol - the GFW has some basic machine learning. If all you do is connect to a specific IP, it'll start throttling connections to said address. It may do the same if it can't scrutinize the encrypted Jabber connection.

The bigger issue is that Jabber is still more complex to set up securely. That's probably why it doesn't have as much mindshare. Given that reputation, and the state of clients across platforms not all implementing the same features (not even equally well), it's harder to convince someone to deal with all of the headaches involved.

I actually had a plane get delayed in Shanghai, and I had no phone plan so the internet was my only option for communication back to the US. Couldn't use Facebook or Instagram. Forget Gmail, because even the Google homepage couldn't be accessed there. Hotmail said it delivered, but I found out the message didn't get received until about 3 weeks AFTER I arrived back in the US. Jabber was the only thing that DID actually work.

By comment on "Signal's open sourced server code hasn't been updated for over a year. Should we be concerned?"

Well, while open source does not mean it's secure, this is still a weird thing to do.

I would simply recommend to stop using Signal and start using XMPP with OMEMO encryption, since this is the gold standard of instant messengers, at least for me. You should especially stop using Signal because it requires your phone number, which immediately disqualifies it for a private messenger.