iPhone can no longer see my MyPassport Wireless Pro Plex Server

I use a WD MyPassport Wireless Pro as my main portable Plex Server. My setup was that I tethered my MyPassport device to the iPhone and then run Plex on IOS on my phone ( which when travelling Bluetooth connects to car ). Plex on IOS would connect to the MyPassport Plex Server over the tethered link. This allows me still to use the phone for Waze etc, plus anything I played would still be scrobbled because the Plex Server had internet via the tethered iPhone. As this is a company phone I don’t want to stream from my home NAS as that uses up data allowance.

I have recently updgraded the IOS to 11.4 and at the same time the Plex player on IOS to version 5.01.

MyPassport still tethers OK to the iPhone ( get the blue bar with 1 connection on the top of the phone ) but the Plex player on the phone fails to talk to the Plex Server on the MyPassport ( reports as being OFFLINE ).

The problem appears to be internet access as the MyPassport device ( despite being connected ) no longer appears to have Internet access through the iPhone.

Any ideas ? Suggestions ?