Linux Mint

this sounds more like network problem than a linux one; but since you're using linux w/o an internet connection, you'll have to use the tools that usually come with Linux:

get a sense of your computer's situation some of these commands will give you the same information and i write them here because i don't know what tools mint provides with its image:

start with basic information. see if your computer has an ip address and who its primary gateway(s) is/are:

ip a
ifconfig -a
netstat -rn
route -n

then use that information to start troubleshooting; these commands are a shortcut; duplicate information; and should be run in order:

for i in $(netstat -rn | grep ^0.0.0.0 | awk '{print $2}') ; do ping -c 3 -W 1 $i ; done
if [ $? -eq 0 ] ; then cat /etc/resolv.conf ; traceroute google.com ; fi

this command provides duplicate information but provided here in case mint has really weird packages on it

for i in $(route -n | grep ^0.0.0.0 | awk '{print $2}') ; do ping -c 3 -W 1 $i ; done
if [ $? -eq 0 ] ; then cat /etc/resolv.conf ; traceroute google.com ; fi

if neither of the last 2 stanzas worked, then your computer doesn't think it's connected to a network and the source of your problem is somewhere with either the hardware, driver or mint.