You want to have two interfaces, one for USB0 and one for ETH2. You cannot do this in the default UI, here is what you should do:

SSH to the router and edit /etc/config/network

You will find the config like below.

config interface 'tethering'
        option proto 'dhcp'
        option ifname 'eth2'
        option metric '30'
        option disabled '0'

You should copy this section and create another section. Give it a new name e.g. tethering1. You can change the metric 30 to other values, e.g. 35 or 50 to give it another priority. The less the value, the more priority.

config interface 'tethering1'
        option proto 'dhcp'
        option ifname 'usb0'
        option metric '30'
        option disabled '0'

Then you should edit /etc/config/firewall and find the following section, and add tethering1

config zone
        option name 'wan'
        option input 'REJECT'
        option output 'ACCEPT'
        option forward 'REJECT'
        option masq '1'
        option mtu_fix '1'
        option masq6 '1'
        option network 'wan wan6 tethering tethering1'

Then reboot your router and it should be OK.

Please note, you should not use the UI to connect tethering later because it may erase your modifications.