!! https://raspberrypi.stackexchange.com/questions/95072/access-point-as-wifi-router-repeater-with-additional-wifi-dongle
!! Access point as WiFi router/repeater with additional WiFi-dongle

- Enable systemd-networkd

For detailed information look at (1). Here only in short. Execute these commands:

# disable classic networking
rpi ~$ sudo -Es
rpi ~# systemctl mask networking.service dhcpcd.service
rpi ~# mv /etc/network/interfaces /etc/network/interfaces~
rpi ~# sed -i '1i resolvconf=NO' /etc/resolvconf.conf

# enable systemd-networkd
rpi ~# systemctl enable systemd-networkd.service systemd-resolved.service
rpi ~# ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

- Configure wpa_supplicant for wlan0 as access point

To configure wpa_supplicant create these files with your settings for country=, ssid=, psk= and maybe frequency= You can just copy and paste this in one block to your command line beginning with cat and including EOF (delimiter EOF will not get part of the file):

rpi ~# cat > /etc/wpa_supplicant/wpa_supplicant-wlan0.conf <<EOF
country=DE
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="RPiNet"
    mode=2
    key_mgmt=WPA-PSK
    psk="verySecretPassword"
    frequency=2412
}
EOF

rpi ~# chmod 600 /etc/wpa_supplicant/wpa_supplicant-wlan0.conf
rpi ~# systemctl disable wpa_supplicant.service
rpi ~# systemctl enable wpa_supplicant@wlan0.service

- Configure wpa_supplicant for wlan1 as client

rpi ~# cat > /etc/wpa_supplicant/wpa_supplicant-wlan1.conf <<EOF
country=DE
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1

network={
    ssid="TestNet"
    psk="anotherSecretPassword"
}
EOF

rpi ~# chmod 600 /etc/wpa_supplicant/wpa_supplicant-wlan1.conf
rpi ~# systemctl disable wpa_supplicant.service
rpi ~# systemctl enable wpa_supplicant@wlan1.service

- Configure interfaces

Create these two files:

rpi ~# cat > /etc/systemd/network/08-wlan0.network <<EOF
[Match]
Name=wlan0
[Network]
Address=192.168.4.1/24
# IPMasquerade is doing NAT
IPMasquerade=yes
IPForward=yes
DHCPServer=yes
[DHCPServer]
DNS=84.200.69.80 1.1.1.1
EOF

Because we don't have a bridge, we need two different subnets. Be aware that the static ip address for the access point wlan0 belongs to another subnet than that from wlan1. For the connection to the internet router we use network address translation (NAT).

rpi ~# cat > /etc/systemd/network/12-wlan1.network <<EOF
[Match]
Name=wlan1
[Network]
DHCP=yes
EOF

Reboot.