WPS 

https://openwrt.org/docs/user-guide/wifi_configuration#wps_options

It is only working, if only these are built in the firmware: hostapd-common wpad-mini, nothing other like that. If not, it will not work.

There were some changes. So this is how it works with 19.07.4.

Minimal steps needed to get WPS running:

In /etc/config/wireless it is important in the wifi-ifaceonly in the 2.4 GHZ.

    option wps_pushbutton '1'

But you can do it with LUCI as well.

opkg update
opkg remove wpad-mini wpad-basic
# if you remove it, it will not work
opkg install wpad hostapd-utils
reboot && exit

# wait about 2 minutes
hostapd_cli -i wlan1 wps_pbc

hostapd_cli wps_pin any $PIN

Debug (not needed) 

reboot && exit

# regenerate the wifi interface
rm -f /etc/config/wireless
wifi config

iw reg get
iw dev

Actual Linksys 3200ACM WPS 

Do what it says above. Then on router on the bottom right there is the WPS button. Click for like 3 seconds or more and it will work.

Example 

config wifi-iface 'default_radio1'
    option device 'radio1'
    option network 'lan'
    option mode 'ap'
    option ssid 'your-ssid'
    option key 'your-secret'
    option encryption 'psk2+ccmp'
    option wps_pushbutton '1'
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