I have a Pi running as a router with it getting an IP address from a phone it's tethered to via usb. On eth1 I'm not sure what settings to put in. I selected eth1 as my interface, but then it got the Pi's ip address from usb0.
pi@raspberrypi4-8gb:~ $ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.220.1 netmask 255.255.255.0 broadcast 192.168.220.255
inet6 fe80::a5c0:39df:8a45:3937 prefixlen 64 scopeid 0x20<link>
ether dc:a6:32:b0:20:8d txqueuelen 1000 (Ethernet)
RX packets 593532 bytes 98095312 (93.5 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 823995 bytes 893290991 (851.9 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
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 1000 (Local Loopback)
RX packets 24987 bytes 1677088 (1.5 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 24987 bytes 1677088 (1.5 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
usb0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.42.80 netmask 255.255.255.0 broadcast 192.168.42.255
inet6 fe80::ca7d:1c58:b2ac:bf81 prefixlen 64 scopeid 0x20<link>
ether fe:7d:a8:5e:8a:ef txqueuelen 1000 (Ethernet)
RX packets 807039 bytes 905252286 (863.3 MiB)
RX errors 41 dropped 0 overruns 0 frame 41
TX packets 487534 bytes 109953220 (104.8 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
wlan0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
ether dc:a6:32:b0:20:8e 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
[i] View the web interface at http://pi.hole/admin or http://192.168.42.80/admin
[i] You may now configure your devices to use the Pi-hole as their DNS server
[i] Pi-hole DNS (IPv4): 192.168.42.80
[i] If you have not done so already, the above IP should be set to static.
[i] The install log is located at: /etc/pihole/install.log
[✓] Installation complete!