Hi,
When I checked the inside of my Librem 13, I found out that one of the antenna cables was pinched in the assembly at about 15cm from the miniPCI adapter. Only the outer insulation was slightly damaged, so no real issues. Now, I don’t know how the Librem 15 is assembled, but you might want to check the integrity of the cables.
Maybe, before opening it, you could try to check what happens if you keep an AP very close (but not less than, say, 2 wavelengths, 25cm) to the antennae. If that doesn’t give you 802.11n, you can most likely rule out a wiring problem.
Cheers
дядя Вова