Gfilui

I freshely installed an OpenVpn on my machine which runs ubuntu server 18.04 LTS. I scanned my ports with nmap it seems to be my ISP filtered all ports so i'm unable to do port forwarding. Is there any way to access OpenVpn outside my LAN without port forwarding.

You need to have a port forward from your public ip address. There is no other way how to connect from outside. Talk to your ISP regarding the port forwarding. Some ISP provide port forwarding free of charge, whereas an extra public IP address costs something.

I don't trust the method you used to determine your ISP "filters all ports so you're unable to do port forwarding". If you haven't set up port forwarding, no ports are going to be open on your IP address, which doesn't necessarily mean your ISP is doing anything.

To port forward, you need to log in to your router (most likely supplied to you by your ISP) and then Google "how to set up port forwarding on <router name/make/model>". For example, for the SuperHub 3:

https://www.google.co.uk/search?q=How+to+set+up+port+forwarding+on+the+SuperHub+3

What port(s) you forward to your Ubuntu 18.04 server to access OpenVPN will depend on how you set up OpenVPN. If you used the default OpenVPN server configuration, you'll have to forward UDP port 1994 to the IP address of your Ubuntu 18.04 server.

You can find out what port OpenVPN is running on by running:

$ sudo ss -plntu
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port 
udp UNCONN 21504 0 127.0.0.53%lo:53 0.0.0.0:* users:(("systemd-resolve",pid=603,fd=12)) 
udp UNCONN 768 0 0.0.0.0:1194 0.0.0.0:* users:(("openvpn",pid=831,fd=7)) 
tcp LISTEN 0 128 127.0.0.53%lo:53 0.0.0.0:* users:(("systemd-resolve",pid=603,fd=13)) 
tcp LISTEN 0 128 0.0.0.0:22 0.0.0.0:* users:(("sshd",pid=852,fd=3)) 
tcp LISTEN 0 128 [::]:22 [::]:* users:(("sshd",pid=852,fd=4)) 

From the above it can be seen that OpenVPN is using UDP port 1194 (in this particular case).

The method you used for testing your ports is not an indication that they are blocked by your ISP.

First, attach an application to listen on the port you want to test. You can do this with netcat from the repository.

$ sudo apt install netcat

Now run this command to test the port you want to verify is not being blocked by your ISP. In this example I'll use the default openvpn port 1194.

$ netcat -l 1194

There are a number of ways that you can test it. You can test it with this website IP address from your phone with wifi turned off so that you will be coming from the outside.

http://yourpublicipaddess:1194

When you access it from the web you will see activity on your screen when the web browser queries it.

You can also use a web tool such as Open Port Check.

Answer the prompt for the port you want to check, then click the Check Port button.

You can use this command to see which ports you have opened (you have a service connected to):

$ netstat -tulnp | grep "LISTEN"

The output will include something similar to this:

(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
 tcp 0 0 0.0.0.0:1729 0.0.0.0:* LISTEN -
 tcp 1 0 0.0.0.0:1194 0.0.0.0:* LISTEN 5454/netcat
 tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN -
 tcp 0 0 0.0.0.0:139 0.0.0.0:* LISTEN -
 tcp 0 0 127.0.0.1:5037 0.0.0.0:* LISTEN 3851/adb
 tcp 0 0 0.0.0.0:32400 0.0.0.0:* LISTEN -
 tcp 0 0 127.0.0.1:32401 0.0.0.0:* LISTEN -
 tcp 0 0 127.0.0.1:5556 0.0.0.0:* LISTEN 4884/chrome

Be sure to use your router's manual for its proper port forwarding configuration.

A quick method to have all traffic directed to your local computer's IP (something for a quick test... could be a serious security issue if left on)

Temporarily turn on Application and Gaming DMZ. Then run the port test described above.

You have those options for having access to your LAN from the outside. If those fail, you would have to communicate with your ISP to unblock the ports you want to use.

Note:

It might be something that happens at times, but I have never seen it, and can't imagine an ISP blocking all ports to their clients. They may block some ports such as port 25 to protect their clients from virus vulnerabilities. Some also block ports that are commonly used for heavy traffic file sharing (often used for pirating software). They often block port 80 to prevent their customers from running heavy traffic web servers. I kind of gave you the wrong port to test in the comments.

You shouldn't find any problems finding an unblocked port using the method above.