Error : Could not resolve archive.ubuntu.com [duplicate]

I am getting this could not resolve issue. I have been on an open network from last two months after fresh installation of ubuntu 18.04. I have just shifted in a LAN with proxy. I have done every settings including updating the /etc/resolv.conf file and have looked at other solutions of these types. None is working.

sudo apt-get update
Err:1 https://download.sublimetext.com apt/stable/ InRelease                     
Could not resolve 'download.sublimetext.com'
Err:2 http://security.ubuntu.com/ubuntu bionic-security InRelease                
Could not resolve 'security.ubuntu.com'
Err:3 http://dl.google.com/linux/chrome/deb stable InRelease                     
Could not resolve 'dl.google.com'
Err:4 http://archive.ubuntu.com/ubuntu bionic InRelease                          
Could not resolve 'archive.ubuntu.com'
Err:5 http://ppa.launchpad.net/leaeasy/dde/ubuntu bionic InRelease     
Could not resolve 'ppa.launchpad.net'
Err:6 http://ppa.launchpad.net/maarten-baert/simplescreenrecorder/ubuntu bionic InRelease
Could not resolve 'ppa.launchpad.net'
Err:7 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Could not resolve 'archive.ubuntu.com'
Err:8 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Could not resolve 'archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Could not resolve 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Could not resolve 'security.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Could not resolve 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Could not resolve 'archive.ubuntu.com'
W: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/InRelease  Could not resolve 'dl.google.com'
W: Failed to fetch http://ppa.launchpad.net/leaeasy/dde/ubuntu/dists/bionic/InRelease  Could not resolve 'ppa.launchpad.net'
W: Failed to fetch http://ppa.launchpad.net/maarten-baert/simplescreenrecorder/ubuntu/dists/bionic/InRelease  Could not resolve 'ppa.launchpad.net'
W: Failed to fetch https://download.sublimetext.com/apt/stable/InRelease  Could not resolve 'download.sublimetext.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.

Try this if your host is behind a proxy server.

Add the following line to /etc/apt/apt.conf. If the file doesn't exist create one.

Acquire::http::Proxy "http://yourproxyaddress:proxyport";

Replace yourproxyaddress and proxyport appropriately.

Or, if authentication is required:

Acquire::http::Proxy "http://user:pwd@yourproxyaddress:proxyport";