I solved it by changing the langugage to English (US), then I could see
the Developer mode option on my Quest. Maybe the language change
triggers an UI update or something, or it is just available in this
langauge (I had it in Spanish (Spain)).
Well I solved it seems. They updated avatar scripts, a lot of them, but
not all the prefabs. Some prefabs that previously worked now they do
not. The only reliable one is LocalAvatar, that works well. And if you
copy the settings of the script OVRAva...
Same problem, sample scenes from Oculus don't work well. Avatar hands
not showing to me. I haven't touched ANYTHING, just installed Oculus
Integration in 2018.4.5.