How to use a custom `resolv.conf` file with systemd-resolved in Ubuntu

Stages

Disable and stop the systemd-resolved service:
```
sudo systemctl stop systemd-resolved
```
Then put the following line in the [main] section of your /etc/NetworkManager/NetworkManager.conf:
```
   dns=default
   rc-manager=symlink
```
In this way we are using dns default in conjunction with rc-managed symlink:

dns=default: NetworkManager will update /etc/resolv.conf to reflect the nameservers provided by currently active connections. Default is used, unless /etc/resolv.conf is a symlink to /run/systemd/resolve/stub-resolv.conf, /run/systemd/resolve/resolv.conf, /lib/systemd/resolv.conf or /usr/lib/systemd/resolv.conf. In that case, systemd-resolved is chosen automatically.

rc-manager=symlink: If /etc/resolv.conf is a regular file, NetworkManager will replace the file on update. If /etc/resolv.conf is instead a symlink, NetworkManager will leave it alone. Unless the symlink points to the internal file /run/NetworkManager/resolv.conf, in which case the symlink will be updated to emit an inotify notification. This allows the user to conveniently instruct NetworkManager not to manage /etc/resolv.conf by replacing it with a symlink.
Delete /etc/resolv.conf
```
rm /etc/resolv.conf
```
Create your own /etc/resolv.conf.custom if you need to customize your DNS
```
nameserver 8.8.8.8
nameserver 10.40.1.40
nameserver 10.40.1.41
```

Make a symbolic link:

sudo ln -s /etc/resolv.conf /etc/resolv.conf.custom

Start the systemd-resolved service:
```
sudo systemctl start systemd-resolved
```

Restart network-manager

sudo service network-manager restart

sudo systemctl restart NetworkManager.service