Our little device now has hardware watchdog support https://github.com/lede-project/source/commit/3fbf3ab44f5cebb22e30a4c8681b13341feed6a6 so @dissent1 I guess kmod-softdog should be removed from target/linux/ramips/image/mt7621.mk
I just did a compile with kmod-softdog removed and it seems like I still have a running and functional watchdog in the router 
My device is running stable both on 2.4Ghz and 5Ghz with all kind of different wireless devices connected (TV, android, iOS, printer, Sonos ...). I'm using the following config for reference.
config wifi-device 'radio0'
option type 'mac80211'
option hwmode '11g'
option path 'pci0000:00/0000:00:00.0/0000:01:00.0'
option htmode 'HT20'
option country 'DK'
option channel '13'
option log_level '4'
option txpower '17'
config wifi-iface 'default_radio0'
option device 'radio0'
option network 'lan'
option mode 'ap'
option ssid '...'
option key '...'
option encryption 'psk2+ccmp'
option wpa_group_rekey '1800'
config wifi-device 'radio1'
option type 'mac80211'
option hwmode '11a'
option path 'pci0000:00/0000:00:01.0/0000:02:00.0'
option htmode 'VHT80'
option channel '128'
option country 'DK'
option log_level '4'
option txpower '17'
config wifi-iface 'default_radio1'
option device 'radio1'
option network 'lan'
option mode 'ap'
option ssid '...'
option key '...'
option encryption 'psk2+ccmp'
option wpa_group_rekey '1800'