NetworkManager handles network connection and periodically checks if an internet connection exist - default 300s.
Disable this by creating a file
sudo cp /usr/lib/NetworkManager/conf.d/20-connectivity.conf /etc/NetworkManager/conf.d
Append the following to the file
echo "interval=0" | sudo tee -a /etc/NetworkManager/conf.d/20-connectivity.conf
Restart the service
sudo systemctl restart NetworkManager
The interval = 0 disables the check. Change the uri to what ever service trusted or controllable.
Consider the implications as NetworkManager is know agnostic on the internet connection.