first you need to go to the installation section of the modem. when you go to the installation section, the modem has a security section (usually, but not known). disable security then create a virtual server-style partition under the transmission header in the same setup settings section (may vary by modem), then create a port from that partition and then create a virtual server. when you do them, the problem disappears. (you need to pay attention when the modem is turned off and on the ip address changes. You can make a fixed ip or you need to constantly check and edit your ip address.
You can find a lot of videos if you searched on the internet as port opening and server creation.If you can't find your own language, search in English.
once you find out, if you encounter this problem again, it will take you 1 minute to solve it. Have fun. https://imgur.com/tudeVop then it's time to start having fun.