I am a new user and I connected the reach M+ to the pixhawk and was unable to power up. I noticed there was a similar problem reported in August 2018 where someone also suggested a solution.
The topic was closed by Emlid, however, the documentation has not been updated, so I guess many people like me are still struggling with this problem.
It would be nice to have the documentation being updated to avoid frustrations like I had. For this issue, it should be necessary to look into the blogs to find a fix.
Elid Reach is not a “low cost” solution and it should be working right out of the box, following the steps as per documentation.