Maybe missing the 5GHz config. You can refer to this config:
# cat /etc/config/wireless
config wifi-device 'radio0'
option type 'mac80211'
option channel '36'
option hwmode '11a'
option path 'pci0000:00/0000:00:00.0'
option htmode 'VHT80'
option txpower '15'
option band '5G'
option disabled '0'
config wifi-iface 'default_radio0'
option device 'radio0'
option network 'lan'
option mode 'ap'
option ssid 'GL-AR750-c8b-5G'
option encryption 'psk-mixed'
option key 'goodlife'
option ifname 'wlan0'
config wifi-device 'radio1'
option type 'mac80211'
option channel '11'
option path 'platform/qca953x_wmac'
option htmode 'HT20'
option hwmode '11ng'
config wifi-iface 'default_radio1'
option device 'radio1'
option network 'lan'
option mode 'ap'
option ssid 'GL-AR750-c8b'
option encryption 'psk-mixed'
option key 'goodlife'
option wds '1'
option ifname 'wlan1'