You will need to use the legacy firmware update mode. the m8p isn’t a very good RTK GPS receiver, realistically float is about as good as it gets, it can do RTK fix but its intermittent at best even under ideal conditions. I spent months trying to get a set of these working when they were first released as they were selling them with the new pixhawk 2 cube that also just came out. I tried everything to get RTK fix but failed and the manufacturer failed to mention that slight issue that it would never get a stable RTK fix so was essentially useless for professional RTK use.
these are much better receviers, they are a fraction of the price and dual band so work as good as a f9p.