No need to be alarmed, to be honest. SSH brute forcing is so common, most admins won't even monitor it ... 
Firstly, as @xize11 said: Disable SSH on your WAN interface and triple check what is going on there.
Second: If you want to be reachable via SSH (which is totally fine!) adjust SSH to work only with public key authentication, see SSH key authentication - #2 by RVer