@Nasdak
Hello, I don't think the issue you are reporting is related to the NAT type of the router, but it is port restricted.
You can check if it requires to open some special ports on the router and ensure UPnP is enabled on the router.
BTW, does the computer connect to the router via LAN cable or by wireless? What are the 5G wireless settings on the router if it is connected via wireless 5G? BTW, what is the brand and make of the wireless card on the computer? Try modifying the wireless settings on the router, such as channel and channel width, you can also try changing the mode to 802.11 a/n/ac mixed, then reboot the router to confirm.