Can we use low gain omni antenna with RU4370 in indoor environment?

It depends.

RU4370 requires the VSWR of the antenna to be lower than 1.8 to avoid excessive power reflections that could damage the device. When applying omni antenna, the VSWR will be around 3, which doesn’t meet the requirement. Thus, the omni antenna should be applied with attenuators. Otherwise, we should replace the omni antenna with plate antenna.