Connect your device either via USB Tethering, Wifi-Hotspot or Ethernet-Hotspot to the GroundPi. USB tethering is more reliable and preferred.
FPV_VR app in Playstore Usage information and source code Leave FORWARD_STREAM=rtp in openhd-settings-1.txt to send a rtp h264 stream to the FPV_VR app. RAW is also possible. Most important is that all settings in the app match those on your GroundPi.
Active thread on rcgroups https://www.rcgroups.com/forums/showthread.php?2873827-FPV_VR-Ultra-low-latency-for-FPV-Virtual-Reality-Android-App