Hello, @Emsanator
Since you mentioned that the IP address was changed and server rebooted, may I know if you have reloaded the daemon and check on the status of the mattermost service?
sudo systemctl daemon-reload
sudo systemctl status mattermost.service
Once confirmed that it was loaded, can you start the service again and share the output?
sudo systemctl status mattermost.service
Also, what is the error that you are seeing on the web interface? That can also help us to identify the potential root cause to it.