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Channels 52-140 require DFS (dynamic frequency selection) by law. They are shared by radar, which is assigned as the primary user of this band and WiFi as a secondary user. Only channels 36-48 are free from this requirement.

A router must constantly monitor for radar signals and if it detects radar impulses on the channel it is using it *must* switch to another, clear channel within something like 60 seconds. You cannot legally operate a wireless router on those channels without DFS being on, and hence, you cannot turn it off, and you cannot override this nomatter what you select. Even if you set a channel manually in this range, it can only "prefer" this channel and cannot unconditionally use it - it must still switch if radar is detected. WiFi is only allowed use those channels as long as they are clear, and not being used by radar.

In an ideal world the router would maintain a list of "clear" channels that are free from radar and switch to the nearest one when the occupied channel gets a radar detection. However when switching channels it has to monitor the new channel for 30-60 seconds to make sure it is actually clear.

In practice it looks like the HH5 is taking the easy option and just defaulting to a guaranteed clear channel, i.e. one that doesn't require DFS, instead of randomly picking a new channel to avoid having to interrupt your connection for up to a minute. This, incidentally, was why the original Superhub couldn't use any 5Ghz channels other than 36-48 - they never implemented DFS at all.