mjhjmtu

I need to access an octopi (raspberry pi) from the Internet. I cannot use a router, so I am using a wifi as my Internet source.

I need to access the raspberry pi using the Internet or any android application using IP and port.

internet > wifi > rasberrypi

What I have tried so far

sudo /bin/su -c "echo -e 'n#Enable IP Routingnnet.ipv4.ip_forward = 1' > /etc/sysctl.conf"
sudo sysctl -p

then

net.ipv4.ip_forward = 1
sudo iptables -t nat -A POSTROUTING -o tun2 -j MASQUERADE
sudo iptables -A FORWARD -i wlan0 -o tun2 -j ACCEPT
sudo iptables -A FORWARD -i tun2 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
sudo iptables -A INPUT -i wlan0 -p icmp -j ACCEPT

sudo iptables -A INPUT -i wlan0 -p tcp --dport 22 -j ACCEPT //should it be any other port?

sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

sudo iptables -P FORWARD DROP //i get error here
sudo iptables -P INPUT DROP// error input,forward is not accepted as parameter
sudo apt-get install iptables-persistent
sudo systemctl enable netfilter-persistent

 sudo netfilter-persistent save

manual port forwarding

sudo iptables -t nat -A PREROUTING -p tcp --dport 666 -j DNAT --to 192.168.0.6
sudo iptables -t filter -A FORWARD -p tcp -d 192.168.0.6 --dport 666 -j ACCEPT
sudo netfilter-persistent save

I followed this website and used wlan0 insted of eth0.
I have changed my IP to static and used it for forwarding.
I don't know which port to use instead of 666? Can I use, say, 1800 or 1993?
I am really new to networking and these commands are new to me.

ip route show command output:

 >ip route show
0.0.0.0/1 via 10.10.0.21 dev tun2
default via 192.168.43.1 dev wlan0 metric 303
10.10.0.1 via 10.10.0.21 dev tun2
10.10.0.21 dev tun2 proto kernel scope link src 10.10.0.22
128.0.0.0/1 via 10.10.0.21 dev tun2
176.126.237.214 via 192.168.43.1 dev wlan0
192.168.43.0/24 dev wlan0 proto kernel scope link src 192.168.43.102 metric 303

do i need control over the openvpn server?