Remote Site not connecting and vice versa

Hi 

We have two servers in the control room and a remote server over a VPN in another location. The are all merged as one site. The workstations in the control room login as normal but are unable to see the remote server (offline red x).  The remote workstation when logged in can its local server but not the 2 servers at the control room.

Port 7001 is reachable in all directions to all servers.  we are using the latest server 6.12.42921 and client versions

I'm not to familiar with how the merged sites are managed, but this just randomly happened one day.

Could you advise?

Thanks