How to fix open port on ubuntu?

Question

iptables

There is a server on ubuntu. I launched an Express backend on it. It's not available. Opened ports:

iptables -A OUTPUT -p tcp --dport 3000 -j ACCEPT
iptables -A INPUT -p tcp --dport 3000 -j ACCEPT

Now everything works.
I restart the server. Everything doesn't work again. nestat shows that port 3000 is not open. I open it again - everything works again.
How can I make sure that the specified port is always open, even after the server is rebooted?
PS on trying to do:
service iptables save
iptables: unrecognized service

Answer 1

iptables does not save its rules when the system is shut down.
for this Wexter advised me to install the iptables rule saver on reboot.
option 2: write the rules in the executable script in the dir /etc/network/if-up.d/ they are executed when the network is turned on and fill the new (after reboot) life of iptables with meaning :)