Check device name

sudo lshw -class network -short

Check hardware information

lshw -C network

Change card LAN

Edit interface

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
 
source /etc/network/interfaces.d/*
 
# The loopback network interface
auto lo
iface lo inet loopback
 
# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

replace eth0 with the interface found with sudo lshw -class network -short.

To disconnect a network

sudo ifconfig eth0 down

To connect a network

sudo ifconfig eth0 up

Script to keep raspberry Pi network alive:

#!/bin/bash
 
LOGFILE=/home/pi/network-monitor.log
 
if ifconfig eth0 | grep -q "inet"
then
       :
       #echo "$(date "+%m %d %Y %T") : Ethernet OK" >> $LOGFILE
else
       echo "$(date "+%m %d %Y %T") : Ethernet connection down! Attempting reconnection." >> $LOGFILE
       ifconfig eth0 up
       OUT=$? #save exit status of last command to decide what to do next
       if [ $OUT -eq 0 ] ; then
               STATE=$(ifconfig eth0 | grep "inet")
               echo "$(date "+%m %d %Y %T") : Network connection reset. Current state is" $STATE >> $LOGFILE
       else
               echo "$(date "+%m %d %Y %T") : Failed to reset ethernet connection" >> $LOGFILE
       fi
fi

Add the script to cron

sudo nano /etc/crontab

Add this line to the end

*/3 * * * * root bash /home/pi/bin/network-monitor.sh

Run every 3 minutes

Check the cron service status

service cron status

Turn on the cron service

sudo service cron restart

If your OS disables cron service by default, you need to run the command sudo service cron start at boot.
To test if the script runs properly, turn down the network and wait for 3 minutes. After that, if you can connect to the Raspberry Pi via SSH, then it runs okay. You can check the log to see more.

Turn the network down

sudo ifconfig eth0 down

Check its log

less /home/pi/network-monitor.log