Since Plex has said they won't make an app for the Quest, we need to utilize the browser to watch items on plex media server. Logging in to that server requires a PIN entry. However the browser doesn't see it as a text entry, so doesn't bring up the keyboard for you to enter the pin. I see no way to manually bring up the keyboard.
While I use plex as an example, it's not the only site I've run in to with this issue.
Annoying problem. But which solution is to clear cache. Sign in again with Plex and the have it remember the user in Plex settings so you don't need pin.