Why are IP addresses of users on my website constantly changing?

I have two websites - one running on cpanel and other one on Amazon AWS Lightsail. On both of them I have Wordpress website and implemented plugin with voting system (exactly the same) and on the first one it is working perfectly - it bans users if they already voted for the picture (checking their ip address from database) but on the second one it is constantly changing IP Address so it can't stop it from voting again.

How can I solve this? How can I set for the users to always have the same IP Address so my plugin will work on both websites exactly the same?

Thanks!