There’s likely a good reason, but it may not be instantly apparent.
Let us know what happens when you “force” the FP2 to use 5GHz.
I have a “work in progress” (i.e. unfinished) wiki on Wi-Fi considerations which among other things mentions a reason for devices not choosing the 5GHz band. The reason could be outdated security option. The case I discuss sounds very similar to yours.