"NAT discovery error: Fail to find an UPNP NAT" when trying to setup LAN server for the first time

[Snufkinson]

I'm trying to make a LAN server to play with my brother on our two laptops. I have an Ethernet cord (I don't think it's a crossover cable but a regular one) and I'm not sure how to do it with just that and no internet.

When trying to do it with internet following this guide: https://wiki.zandronum.com/Personal_Gam ... Doomseeker I got this error: "NAT discovery error: Fail to find an UPNP NAT." It's the same error as this guy: https://zandronum.com/forum/viewtopic.php?f=39&t=3234 who apparently solved it but didn't have the decency to post his solution. What am I doing wrong? Am I on the right track to playing a co-op LAN game of doom2? Is it possible to play over just an Ethernet cord connecting the two laptops and no internet or router?
Thanks in advance.