Android phone as gateway

share_lte.sh

#!/system/bin/sh
# Share LTE over WiFi STA, use when AP is not possible.
# You have to make sure the phone can connect to LTE and WiFi at the same time,
# e.g. keep portal detection failing.
RMNET_DEV=$(ip route|grep rmnet|cut -d " " -f 3)
WLAN_SUBNET=$(ip route|grep wlan|cut -d " " -f 1)
WLAN_DEV=$(ip route|grep wlan|cut -d " " -f 3)

iptables -F natctrl_FORWARD
echo 1 > /proc/sys/net/ipv4/ip_forward

IPTABLES_MASQ_RULES=$(iptables -t nat -nL POSTROUTING | grep MASQUERADE)
if [ -z "$IPTABLES_MASQ_RULES" ]; then
    iptables -t nat -A POSTROUTING -s $WLAN_SUBNET -j MASQUERADE
fi

ip route add table $RMNET_DEV $WLAN_SUBNET dev $WLAN_DEV

share_tun.sh

#!/system/bin/sh
# Share VPN over WiFi STA, use when AP is not possible.
# You have to make sure the phone can connect to VPN and WiFi at the same time,
# which means there must be ways other than WiFi to establish the VPN connection.
# In this way you get 3 transports LTE/VPN/WiFi active at the same time.
# You may even try to extend the rules to get all possible transports like
# USB/BT/WiFi/LTE/VPN active simultaneously :-)
WLAN_DEV=$(ip route|grep wlan|cut -d " " -f 3)
LAN_HOST=192.168.1.10
TUN_DEV=tun0

iptables -F natctrl_FORWARD
echo 1 > /proc/sys/net/ipv4/ip_forward

IPTABLES_MASQ_RULES=$(iptables -t nat -nL POSTROUTING | grep MASQUERADE)
if [ -z "$IPTABLES_MASQ_RULES" ]; then
    iptables -t nat -A POSTROUTING -s $LAN_HOST -j MASQUERADE
fi

ROUTE_TEST_RESULT=$(ip route get 8.8.8.8 from $LAN_HOST iif $WLAN_DEV)
if [ -z `echo "$ROUTE_TEST_RESULT" | grep "table 61"` ]; then
    ip rule add from $LAN_HOST lookup 61
fi

ip route add table 61 $LAN_HOST dev $WLAN_DEV
ip route add table 61 default dev $TUN_DEV
ip route add table local_network $LAN_HOST dev $WLAN_DEV

Hi. Not sure if my notes here might help you: https://gist.github.com/gcleaves/ec7a06f8c0bd436c1bc2eb922a246d26