from that sites it works fine on the two devices I have, unlike if I try to access from the localhost
Okay, good to know, in that case it sounds like your device does support WebXR.
Are you unable to launch the AR view when the example is running on your PC and your mobile device is connected via USB to your PC?
Does the workaround described above improve your experience?
If you are using the Chrome WebXR emulator extension (or the Firefox equivalent), when you build the example and access at localhost are you then able to launch into AR as expected?