Remote Server Unavailble on Mac PMS but accessible through iphone app

Hi,
I'm running most recently updated version of PMS (1.0.0.2261) and usually access through the app on safari on my mac (OSX El Capitan). I access 2 friends' servers remotely and have done so for over 2 years without issue. I know the servers are online (as I can access the content on the iphone app) but for the last 4 days it just says 'server unavailable'. i haven't changed anything about the setup of the router or internet connection and it runs via ethernet. the phone works over the same connection via wifi.
i have tried disconnecting the router and resetting and then restarting the PMS but apart from that i have no idea where to start to troubleshoot the problem. its the same when trying to access it from the macbook.
There is no issue with accessing my content either remotely with the laptop or via the computer itself, its just the 2 friends' remote servers.
many thanks