Unable to ping or connect to internet via WLAN

Problem statement

Hi . I needed to auto-connect my system to another system via Ethernet so I assigned both these systems respective static IPs. On my host system, I did this by editing the /etc/network/interfaces.d/eth0 as:

auto eth0
iface eth0 inet static
address 10.0.0.1 
netmask 255.0.0.0
gateway 192.168.1.254

Meanwhile the /etc/network/interfaces file sourced this file as

# interfaces(5) file used by ifup(8) and ifdown(8)
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

This worked as intended. For internet I used WLAN0 but now, when I changed my router hardware, I asked my host system to pair with this new router via nmcli (Headless mode). But now the internet won't work, nor am I able to ping 8.8.8.8 or google.com.

Additional verbose

ifconfig spits out:

eth0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 10.0.0.1  netmask 255.0.0.0  broadcast 192.168.0.255
        ether 00:04:4b:cb:ab:ad  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 40  

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1  (Local Loopback)
        RX packets 675  bytes 51512 (51.5 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 675  bytes 51512 (51.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

rndis0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 1a:af:98:a3:44:39  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

usb0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 1a:af:98:a3:44:3b  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.1.100  netmask 255.255.255.0  broadcast 192.168.1.255
        inet6 fe80::2123:f14d:ba67:f32  prefixlen 64  scopeid 0x20<link>
        ether 0c:54:15:b6:63:9f  txqueuelen 1000  (Ethernet)
        RX packets 92892  bytes 29709877 (29.7 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1593  bytes 258271 (258.2 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

route spits:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         _gateway        0.0.0.0         UG    0      0        0 eth0
default         _gateway        0.0.0.0         UG    600    0        0 wlan0
10.0.0.0        0.0.0.0         255.0.0.0       U     0      0        0 eth0
link-local      0.0.0.0         255.255.0.0     U     1000   0        0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U     600    0        0 wlan0

How can I connect to the internet ?

eth0 is connected to a network (10.0.0.0/8) which cannot provide a route to the internet so it should not be in your routing table.

Try removing that route, something along the lines of :

    sudo route delete default gw _gateway dev eth0

Update your interfaces to remove the gateway as it is not accessible from the 10.0.0.0 network.