In this example I've forked the example by Klaus Weidner (https://arimg.glitch.me/) and implemented a 360° portal for A-Frame.
This uses the new marker tracking feature in Chrome on Android. Currently requires Chrome Canary 88.0.4313.2 or newer, and needs "WebXR Incubations" enabled in chrome://flags.
You can read more about this feature here: https://github.com/immersive-web/marker-tracking/blob/main/explainer.md
WARNING: Experimental. This is a preliminary API and is likely to change.
Check out the Twitter thread here: https://twitter.com/stspanho/status/1323566664470900737