error code DFPLN4000

Trying to add unlimited data to my plan (using my own modem). Ran into the same problem that many other users have had over the years getting error code DFPLN4000 when trying to add it. It is absurd business policy to:

A. Not have already fixed this obviously common problem

B. Not have easily accessible human customer support to allow customers to add this quickly (the AI support is useless for this, just kept sending me the same link over and over and wouldn't transfer me to an agent)

C. Require people to join a forum AND make a post asking the same question that dozens of people have already asked just to be invited to direct message a support person to get this service added. 

Will certainly message a support person when they respond to me, but will also be researching other internet options in the meantime because this is just ridiculous.