Allow me to understand the requirements based on the information you provided here:
My company have 5 office that we would like to setup separate 5 mattermost servers
However, all office was setup the servers
You have 5 different installation of Mattermost Server for each office.
how can i connect them at 1 channel or server to server connection?
You want to have all the offices to be able to communicate and collaborate under a single umbrella. For example:
All users should be able to send messages, send attachments, utilize integrations, have audio/video calls in one main channel.
In this case, there is no direct way of doing this in such a way that you would like to consolidate the 5 instances together based on the answer shared in this post.
The potential, unofficial method is to migrate these contents and data into a single deployment and allow access for users from these offices. Tricky process.