SSH keeps disconnecting after few minutes of inactivity

Some over-zealous routers like to drop TCP connections that are idle for too long (i.e. don't transmit any data). It might because they assume the user only uses things like HTTP, where the connection is often closed after a single query is complete.

Assuming OpenSSH, use the ClientAliveInterval and ClientAliveCountMax directives in sshd_config, or equivalently ServerAliveInterval and ServerAliveCountMax in the client side config (~/.ssh/config or /etc/ssh/ssh_config) to enable protocol-level keepalive packets.

They're actually meant to detect if the remote host has gone away, but since they cause messages to be sent when the connection is otherwise idle, they also work to prevent the connection from being seen as idle by outside devices.

*AliveInterval interval after which the client/server sends a query to the remote, and *AliveCountMax sets the number of unanswered queries after which the the client/server drops the connection as inactive.

Something like these values should do:

ClientAliveInterval 15
ClientAliveCountMax 4