pfSense

using `ifconfig -v <wirelessinf> list chan` possible to see supported HT modes:

# ifconfig -v rtwn0_wlan0 list chan
Channel   1 : 2412      MHz 11b          Channel   7 : 2442      MHz 11g ht/20    
Channel   1 : 2412      MHz 11g          Channel   8 : 2447      MHz 11b          
Channel   1 : 2412      MHz 11g ht/20    Channel   8 : 2447      MHz 11g          
Channel   2 : 2417      MHz 11b          Channel   8 : 2447      MHz 11g ht/20    
Channel   2 : 2417      MHz 11g          Channel   9 : 2452      MHz 11b          
Channel   2 : 2417      MHz 11g ht/20    Channel   9 : 2452      MHz 11g          
Channel   3 : 2422      MHz 11b          Channel   9 : 2452      MHz 11g ht/20
...

with `ifconfig <wirelessinf> channel N:ht/WIDTH` you can set the desired channel width, i.e.

# ifconfig rtwn0_wlan0 channel 1:ht/20

channel number
         Set a single desired channel.  Channels range from    1 to 255, but
         the exact selection available depends on the region your adaptor
         was manufactured for.  Setting the    channel    to any,    or - will
         clear any desired channel and, if the device is marked up,    force
         a scan for    a channel to operate on.  Alternatively    the frequency,
         in    megahertz, may be specified instead of the channel number.

         When there    are several ways to use    a channel the channel num-
         ber/frequency may be appended with    attributes to clarify.    For
         example, if a device is capable of    operating on channel 6 with
         802.11n and 802.11g then one can specify that g-only use should
         be    used by    specifying ``6:g''.  Similarly the channel width can
         be    specified by appending it with ``/''; e.g., ``6/40'' specifies
         a 40MHz wide channel, These attributes can    be combined as in:
         ``6:ht/40''.  The full set    of flags specified following a ``:''
         are: a (802.11a), b (802.11b), d (Atheros Dynamic Turbo mode), g
         (802.11g),    h or n (802.11n    aka HT), s (Atheros Static Turbo
         mode), and    t (Atheros Dynamic Turbo mode, or appended to ``st''
         and ``dt'').  The full set    of channel widths following a '/' are:
         5 (5MHz aka quarter-rate channel),    10 (10MHz aka half-rate    chan-
         nel), 20 (20MHz mostly for    use in specifying ht20), and 40    (40MHz
         mostly for    use in specifying ht40).  In addition, a 40MHz HT
         channel specification may include the location of the extension
         channel by    appending ``+''    or ``-'' for above and below, respec-
         tively; e.g., ``2437:ht/40+'' specifies 40MHz wide    HT operation
         with the center channel at    frequency 2437 and the extension chan-
         nel above.

see also https://forum.netgate.com/topic/83312/question-about-clone-interface-20mhz-width