I wasn't able to get the UndeadNights mod to work on my Aternos server. I added the following mods to the mods folder in appdata: modmenu-13.0.2.jar, UndeadNights-0.9.5-Fabric-mc1.21.3.jar, and fabric-api-0.118.0+1.21.4.jar. I also added the mods in the Aternos server through their website. But I when I play the server I don't see any difference; I see that the mod is added and I am to configure it but I there are no hordes and messages in chat.

Did you configure the mod on the server itself? Modmenu only works in singleplayer.
For multiplayer with a stand alone server, you will have to edit the config file via the Aternos website on the server. You need to navigate to config/undeadnights.json5

Oh. I got it to work. Thank you so much!! I love this mod

No problem. I'm glad to hear that you got it to work and that you like my mod :)