For those with this problem, not all hope is lost.
Here is an update for our situation -be sure to read the words of caution at the end of this message.
To start with, I was not able to log in as usual.. I tried requesting a new password and was denied: no account with that email.

So I re-registered with the same email address and successfully logged in. The !Success problem was gone

but none of our phones showed up.
Then I tried adding a phone. Each phone was "associated with another account" and could not be added.

So I went to the technical support request form, which requires the phone's serial number. Each phone was "not in our system". !!

These are Net10 phones we use every day.
So, I grit my teeth, and called customer service. Sure enough, I spent one and a half hours, including getting disconnected.

But the good news is that they were able to put two our phones on the account.

Words of caution: (I'm conjecturing some of this)
Both phones were "associated" with other email addresses. Moral: don't tell a customer service rep any address other than the one you use to login to the My Account online.
The phone we could not add is an Unlimited phone.
Also, I have seen in this forum that others had the !Success problem go away for a time only to have it return. We'll see.
BTW - if any of my previous requests have led to these changes, no one contacted me about it.