Apologies if you're having difficulties adding your Nest Hub 2nd Gen to your speaker group. Let us help you out. Were there changes to your Wi-Fi? What is the error message when linking it back to your speaker group? Are all of them on the same network? It would help a lot if you could do a sequential reboot first. Unplug the power cord of your router followed by your Nest speakers. Plug them back in after 3 minutes tops. Restarting your phone might also help. If the issue persists, how about if we delete and recreate a new speaker group. Check out this guide for better information.
Keep us posted.