Welcome to the Auth0 Community!
First, the documentation you shared is the correct doc on using a custom SMS gateway provider other than Twilio for Passwordless login.
Now, if you’re having issues with the Passwordless form UI, then the problem is likely to do with the Universal Login Lock (passwordless) template.
After testing this myself, I was able to set up a custom SMS gateway and render the login page correctly. See below.
In this case, could you please try using the default Lock (passwordless) template to see if you can see the page correctly?
Looking forward to your reply.
Thanks.