This has been an issue for months now. Whatever you do do not factory reset, get a new sim, or change your phone number, it will not fix the issue. Samsung messages chat has not worked for since the 2nd Jan. It worked after the number change just for 8 days only. I've managed to connect to Google chat twice. The first time I did I set Samsung to default and chat was not working and then Google wouldn't verify. Seems when you change to Samsung it messes it up for Google when you finally get to connect. Send an error log to Samsung as I'm sure its Samsung messages. They keep telling me to contact my network and its not my network.
