Try this:
HT Caps:
[LDPC][HT40+][SMPS-STATIC][SHORT-GI-20][SHORT-GI-40][TX-STBC][RX-STBC1][MAX-AMSDU-7935][DSSS_CCK-40]
VHT Caps:
[MAX-MPDU-11454][RXLDPC][SHORT-GI-80][RX-STBC-1][RX-ANTENNA-PATTERN]
/var/ipfire/wlanap/hostapd.conf
driver=nl80211
######################### basic hostapd configuration ##########################
#
interface=blue0
country_code=DE
ieee80211d=1
ieee80211h=1
ieee80211w=0
channel=36
hw_mode=a
ieee80211ac=1
ieee80211n=1
wmm_enabled=1
ht_capab=[LDPC][HT40+][SMPS-STATIC][SHORT-GI-20][SHORT-GI-40][TX-STBC][RX-STBC1][MAX-AMSDU-7935][DSSS_CCK-40]
vht_capab=[MAX-MPDU-11454][RXLDPC][SHORT-GI-80][RX-STBC-1][RX-ANTENNA-PATTERN]
vht_oper_chwidth=1
vht_oper_centr_freq_seg0_idx=42
logger_syslog=-1
logger_syslog_level=0
logger_stdout=-1
logger_stdout_level=4
dump_file=/tmp/hostapd.dump
auth_algs=1
ctrl_interface=/var/run/hostapd
ctrl_interface_group=0
ssid=YOUR_SSID
ignore_broadcast_ssid=0
######################### wpa hostapd configuration ############################
#
wpa=2
wpa_passphrase=YOUR_SECRET_PASSWORD
wpa_key_mgmt=WPA-PSK
rsn_pairwise=CCMP
wpa_group_rekey=600
/var/ipfire/wlanap/settings
DRIVER=NL80211 COUNTRY=DE CHANNEL=36 INTERFACE=blue0 TXPOWER=auto NOSCAN=off HW_MODE=ac MACMODE=0 CLIENTISOLATION=off VHTOPSEG0=42 GTKREKEY=600 IEEE80211W=0 PWD=YOUR_SECRET_PASSWORD VHTOPCHW=1 DEBUG=4 SYSLOGLEVEL=0 ENC=wpa2 SSID=YOUR_SSID VHTCAPS=[MAX-MPDU-11454][RXLDPC][SHORT-GI-80][RX-STBC-1][RX-ANTENNA-PATTERN] HIDESSID=off APMODE=on WPAREKEY=600 HTCAPS=[LDPC][HT40+][SMPS-STATIC][SHORT-GI-20][SHORT-GI-40][TX-STBC][RX-STBC1][MAX-AMSDU-7935][DSSS_CCK-40]
After editing the two files, don’t change a field in the webgui !! Only restart is allowed, otherwise the files will be overwritten.