Why does it say Cant Resolve hostname when i add in an aternos server (note i triple checked the ip and it was correct)

i tried to relaunch MC and it didnt work i triple checked the ip and it was correct i did everything i could and for some reason i still couldnt play on my server it always says unknown host when i try to join and it always says cant resolve hostname at the server menu, if someone could please help me with this i would be so grateful (note: its a modded server... but i tried other servers as well and it still didnt help)