You can have as many as you like. It picks one at random to use:

MasterServerList=(Address="utmaster.openspy.net",Port=28902)
MasterServerList=(Address="ut2004master.333networks.com",Port=28902)
MasterServerList=(Address="ut2004master.errorist.eu",Port=28902)

Servers can choose to report to as many as they like (with the right serveractor)

Just to be sure:

If you go to Ut2004\system directory
Open UT2004.ini

Under [IpDrv.MasterServerLink] (There can only be 3 lines)
LANPort=11777
LANServerPort=10777
MasterServerList=(Address="utmaster.openspy.net",Port=28902)

Like I mentioned: In the news tab should be de openspy news


It might be useful if you join Discord:
https://discord.gg/Uu9XknSj

In the news tab you should see the openspy news.
It is not shown in your recording,  UT is trying to reach the old server: 

In the game, open the console (~ key) and paste the following command and run it:
set IpDrv.MasterServerLink MasterServerList ((Address="utmaster.openspy.net",Port=28902))
and relaunch the game. This will update your game automatically.


https://ut2004serverlist.com/how-to-update-your-game/

This will be changed in the UT2004.ini

[IpDrv.MasterServerLink]
LANPort=11777
LANServerPort=10777
MasterServerList=(Address="utmaster.openspy.net",Port=28902)      <----- This is the new master server


Check:
You should see the servers in the internet tab
https://www.youtube.com/watch?v=n7tgHD1MzME

If not I think there is a rights issue. (you can not write in the unreal directory)

Thanks for the information. First, I added the ut2004.ini file with the commands indicated. This is what that file contains, and I only added the information you provided me:


[Core.System]
Suppress=DevLIPSinc

[Engine.GameEngine]
ServerPackages=OnslaughtBP
ServerPackages=AssaultBP

[Editor.EditorEngine]
EditPackages=OnslaughtBP

[Engine.LevelInfo]
MaxClientFrameRate=+90.0

This is what I added.

First version:
[IpDrv.MasterServerLink]
LANPort=11777
LANServerPort=10777
MasterServerList=(Address="ut2004master1.epicgames.com",Port=28902)
MasterServerList=(Address="utmaster.openspy.net",Port=28902)

the second version:

[IpDrv.MasterServerLink]
LANPort=11777
LANServerPort=10777
MasterServerList=(Address="utmaster.openspy.net",Port=28902)

It didn't work even though I clicked "update" in the program folder. The unofficial servers still don't appear.

In the news tab you should see the openspy news.
It is not shown in your recording,  UT is trying to reach the old server: 

In the game, open the console (~ key) and paste the following command and run it:
set IpDrv.MasterServerLink MasterServerList ((Address="utmaster.openspy.net",Port=28902))
and relaunch the game. This will update your game automatically.


https://ut2004serverlist.com/how-to-update-your-game/

This will be changed in the UT2004.ini

[IpDrv.MasterServerLink]
LANPort=11777
LANServerPort=10777
MasterServerList=(Address="utmaster.openspy.net",Port=28902)      <----- This is the new master server


Check:
You should see the servers in the internet tab
https://www.youtube.com/watch?v=n7tgHD1MzME

If not I think there is a rights issue. (you can not write in the unreal directory)

Is it working yet?

Well, I've followed the various tips. I've already copied all the UT2004 files from the PC that allow me to connect to the MIA server; but some unofficial servers don't appear in the game's Internet tab. I made this video to show what my tabs look like. And how, when I try to open the MIA server address, the game starts loading but stops. It could be that I need to connect closer to the modem, because it says the connection is lost. I played a little game to show you that the game works individually.