The problem seems my (old) smartphone. The interesting thing is that browsers are up to date on both devices (I tested both Chrome and Firefox), but on my smartphone (both browsers) is not working, while everything is OK on the other one.
My theory: is not about the software, but something at OS level. Is it possible that is something about native WebGL stuff?
Huh, maybe try visiting WebGL Report on the smartphone in question. It should let you know the basic support level of WebGL features. If you drop a screenshot here, we could potentially help ID the issue.