Unable to connect to server from iphone




I'm totally baffled. I've got a PLEX server running on a WDEX2100 NAS. I have apps on various devices, all working fine. The problem I have is on one device, my wife's iPhone 6, whereby the server is showing as unauthorized. This hasn't always been the case, it used to work just fine.
Today I have waded through various forums & guides & found myself in an even worse position - something I read on this site suggested signing out of the app & back in. Neither of us can remember the sign-in details & when I requested a password reset the email with instructions didn't arrive. So, not only can we not access the server on this particular device, now we're not signed in to the app & don't know how to remedy this!

Can anyone help, with:

1 - Gettting a password reset & signing back in to the app
2 - Sorting the issue of the server being unauthorized.

Many thanks...