The Plex Tuner Service says:
Jun 01, 2017 20:31:37.747 [0x7f6cf4a1f700] INFO - * v4l::DeviceGetList_impl. getting v4l device list
Jun 01, 2017 20:31:37.748 [0x7f6cf4a1f700] WARN - [E] v4l::DeviceGetList_impl. open failed on frontend /dev/dvb/adapter0/frontend0 (13)
Jun 01, 2017 20:31:37.748 [0x7f6cf4a1f700] WARN - [E] v4l::DeviceGetList_impl. open failed on frontend /dev/dvb/adapter1/frontend0 (13)
Which means Plex doesn’t have permission. Try doing ‘sudo usermod -a -G video plex’ to give the plex user access to the video group, which is likely where /dev/dvb/adapter1/frontend0 lives and you should be good to go.
