Note: iOS 13 removed this setting. Instead, you'll be asked to provide access to your device's controls each time you load a VR scene. The fix for the iOS 13 issue was released on November 27th.
As of iOS 12.1, Apple added ‘Motion & Orientation Access’ setting for Safari. By default, the setting is turned off & prevents access to device’s accelerometer and gyroscope, which means that ThingLink and other online VR/AR items may have troubles working on your device.
To enable accelerometer for ThingLink on your iOS device, please go to ‘Settings’ – ‘Safari’ and enable ‘Motion & Orientation Access’.