I am using Ubuntu 20.04 with plain GNOME UI on a single pc - Intel-NUC.

My Internet is down to 8MBs, but on my mobile phone, using the wifi of the same router I get 45MBs.

I researched this and found out to change the config file:
sudo nano /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf

And when I try, I get the error:

sudo: error in /etc/sudo.conf, line 0 while loading plugin "sudoers_policy"
sudo: /usr/lib/sudo/sudoers.so must be only be writeable by owner
sudo: fatal error, unable to load plugins

What can I do about that? I searched this and some comments are saying - depending how did you installed Ubuntu... What does this mean? From an ISO on a bootable USB stick.

In the machine setup->Network Ethernet(eno1) Connected - 1000 MBs
Ethernet (enp5s0) Cable unplugged, the switch is turned OFF

My PC is connected by cable to the router on my desk.

Help please

Thanks
Alex

You have somehow borked sudo. That makes administration more difficult. On Ubuntu, if there is no root account, perhaps impossible. I'm not at all proficient with Ubuntu, so I'll refrain from suggesting anything.

Where do you get the 8MB/sec figure? An individual file download, speed test, or what?

I am testing it on the web with a site of our service provider.
Had to start again, reinstalled Ubuntu, the 6th time....

Cheers
Alex