ip is assigned, local netowrk works but the external network is unreachable

I am using mint18.3 on my system which is based on ubuntu 16.04. The system is connected to the internet via an ethernet cable. The cable is attached and works but the connection is detected 1 month ago. The problem is that when I attempt to connect to any websites other than the local ones, it fails. ping 8.8.8.8 and ping www.google.com give connect: network unreachable error. The following is the result of the ifconfig command:

enp4s0    inet addr: 192.168.197.18 Bcast: 192.168.207.255 Mask: 255.255.240.0
lo        inet addr: 127.0.0.1 Mask: 255.0.0.0

Contents of the /etc/resolv.conf and /etc/network/interfaces are reported in the following:

/etc/resolv.conf

nameserver 81.31.160.34
search sharif.ir

/etc/network/interfaces

auto lo
iface lo inet loopback

auto enp4s0
allow-hotplug enp4s0
iface enp4s0 inet dhcp

Also, I checked the routing table with the command ip route show. It reported multiple routes as in the below. I am new to network issues and I cannot understand them well.

81.31.160.0/19 via 192.168.192.1 dev enp4s0
169.254.0.0/16 dev enps4s0 scope link metric 1000
172.16.0.0/12 via 192.168.192.1 dev enp4s0
192.168.192.0/20 dev enp4s0 proto kernel scope link src 192.168.197.18
213.233.160.0/19 via 192.168.192.1 dev enp4s0

How can I solve the connection issue?

ip route add default via 192.168.192.1 dev enp4s0