I'm not certain, but I believe that setting your server computer to have a fixed IP address on your network and then setting your router to send incoming requests for UDP port 5121 (the default port for NWN servers) to the IP address you set for your server should solve this problem. You could try also forwarding the other ports which are used by NWN to the game server's fixed IP address.
Sometimes, you can set the DHCP range so that it excludes the IP address which you set the server to, but some routers may require it to be included in the DHCP range. If the latter, put the address at the top of the DHCP range so that it's unlikely to ever be allocated by DHCP. Confusingly, some routers will choke on having a fixed address allocated within their DHCP range and not explicitly excluded from DHCP by another setting somewhere.
Download the newest server client, released in january 2025 (37-14), it fixes the UDP Punch through issue. Extract it to your bin folder, overriding the 37-13 server binary (which was part of the last patch). You can find the binaries here: https://nwn.beamdog.net/downloads/
Servers hosted on static IP's seems to be unaffected, there are several older versions of the server client up and running with no connection issues. See https://nwn.beamdog.net/ for version details on running servers, some run .35 or .36, there are even .34 versions still running.
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Sometimes, you can set the DHCP range so that it excludes the IP address which you set the server to, but some routers may require it to be included in the DHCP range. If the latter, put the address at the top of the DHCP range so that it's unlikely to ever be allocated by DHCP. Confusingly, some routers will choke on having a fixed address allocated within their DHCP range and not explicitly excluded from DHCP by another setting somewhere.
Servers hosted on static IP's seems to be unaffected, there are several older versions of the server client up and running with no connection issues. See https://nwn.beamdog.net/ for version details on running servers, some run .35 or .36, there are even .34 versions still running.