I may have figured it out and it is a similar situation to the original poster.
If you go into the settings for your modem, CenturyLink includes a “cyber security” function in the advanced settings of the modem, something which I had not ever noticed before.
From what I have been reading, it seems to be a basic firewall run off of McAfee software. After reading other unrelated forums in which people complained about how this setting blocked functionality of various programs that require the ability to communicate outside the home network (things like email and dns servers) and how this problem was solved by turning this function off, I have been running a trial of having it off. Since then, I have not run into any issues with Plex and internet connectivity.
I’m guessing that either, like the OP, a new definition was pushed out and blocked Plex or, equally plausible, a new definition is needed due to a change in Plex and it’s not coming from CenturyLink (esp. given their general lack of hardware support and the ongoing changeover in ownership).
There doesn’t seem to be any way to force an update or change the definitions that I can see, and thus the two options are to have it on or off. Provided you are running firewall software on your devices, I don’t imagine the risk is incredibly high for normal users to turn this function off, but of course it’s up to you.
