My progress bar froze?
And even when it did, clients are conditioned to expect progress bars to freeze and then suddenly finish, from Windows and lots of other software.
This is a solved problem and its solution isn’t hard to implement. IMO not doing it the right way is an insult to your users.
Unless you have access and/or reimplement the endpoint, there's no "solution" and it's not a "fixed problem".
The most true thing would be to just show a label "Query send, waiting for results. Don't know how much it will take".
The indeterminate progress bar/spinner were invented decades ago for this use case. There’s no need to assume users are “conditioned to expect progress bars to freeze and then suddenly finish”, and reinforce that.
If you don’t know when bytes will start arriving or how many will arrive, don’t pretend you do know.