Hi,
I assume that you use SeleniumLibrary…
(It is always important to also tell us which Library you use, because Robot Framework itself can not automate WebBrowser. Libraries can)
With SeleniumLibrary one deprecated known possibility is, to send the user and pwd as url, what you already tried!
You are right. Basic Auth are not alerts.
You can Google for Basic Auth Selenium but there are pretty old answers. Not sure if they are still working.
One solution would be to leave SeleniumLibrary behind and switch to Browser library.
That brand new modern library has the possibility to add Basic Auth headers to a new Context.