HDOP means horizontal dilution of precision, which is a measure of how accurate your GPS signal is. It is related to the number of satellites you are connected to and how far apart they are. The further apart the more accurate your position is.
So it sounds like the area you are flying in might not have the best reception. Could you share a photo of how you mounted your GPS module as that can also be the problem if it doesn't have a clear view of the sky