On the host, try doing a chmod -R 777 /dev/dvb That should make permissions wide open for it.
Also, there are reports of the latest plex version not recognizing dvb cards, but downgrading to the previous version makes it work. There are some forum posts on plex about it, like this one: https://forums.plex.tv/t/plex-1-16-2-unable-to-load-dvrs-after-1-july-hangs-when-trying-to-add-new-dvr/430071