1-open terminal and add the command:[if u have installed gedit text editor use it, if not use nano instead of gedit]
sudo gedit /etc/resolv.conf
2-u will append that line in it:
nameserver 192.168.1.1(note there is space between nameserver and ip,note u will replace that with ur primary dns, u can get it from connection info from icon of ur network).
3-after save , the connection will work good, but if restart the computer, u will get the same problem again
4-to solve it premently, open protonvpn again, that should solve the problem