DIAGNOSIS Bluetooth pairing failure between the A3500 base and your mobile device. This is typically a proximity or interference issue.
FIX Follow this sequence. Ensure the blender is plugged in and the touchscreen is on. On your phone, turn Bluetooth off, then on. Delete the Vitamix app. Reinstall the latest version from the official app store. Open the app. Place your phone within 3 feet of the blender base. Initiate pairing from the app. The blender's touchscreen will show a pairing code. Confirm it matches the app.
IF THAT DIDN'T WORK The blender's Bluetooth module may need a reset. Unplug the blender for 5 minutes. While unplugged, press and hold the power button for 30 seconds. Re-plug. Also, disable any VPN or ad-blocking software on your phone, as these can block the local connection the app requires.
HEADS UP The blender must be in standby mode (plugged in, screen on) to connect. It cannot connect while actively running a blend cycle. The app is for recipe guidance only; it does not control blender functions remotely for safety reasons.