I took this from the support page:

Unfortunately Neverwinter Nights 1 and 2 master servers are no longer functional. Please use the Direct Connect option to log onto your preferred server via IP address.

In order to do that firstly go to your GOG account, in order to get unique cdkeys:
https://gog.com/account
(Select Neverwinter Nights 2 -> More -> Serial Keys)

After that please make sure to run Serial Tool as administrator, to input your unique cdkeys. You can find this application in the game's installation folder (SerialTool executable), or if you installed the game via Galaxy, open Galaxy, select the game, click on More -> Other -> Multiplayer Key Changer.

Start the game, click on Multplayer -> type any username/password, and click on Sign In. The server-gate will be skipped, but you should still be able to connect to multiplayer servers, by using Direct Connect option.

The current list of active servers can be found here: http://www.nwnlist.com/

Additionally we highly recommend using Neverwinter Nights 2 Client Extension:
https://neverwintervault.org/project/nwn2/other/nwn2-client-extension
It restores some in-game functionality, as well as repairs some minor bugs. After installing it please run the game directly using NWLauncher executable.

Just to add, NWN Enhanced Edition has a working online lobby that doesn't need any tweaking nor direct connection. So if you're planning to play a lot of multiplayer in NWN1, it might be wise to buy the EE.