firewall is not even activated yet its a new server
i have done this many times before and i never had this problem
i have to mention that i have installed mongodb on my own laptop and its working well but when i try sudo apt-get update iam having the same error
iam not sure about internet ; there are too many restrictions on internet here (in iran) but i tried with 2 different internet providers and also with vpn
i have tried what was said in the link you mentioned the result was the same still same issue exists