Get online support
Could you try turning off wifi on both routers; button on top of unit. On the slave, use your computer to change the ip address (dot 10), reconnect to new ip address, disable dhcp on slave, save and reboot. While slave is rebooting, connect computer to main unit, connect cable between units.
When slave is up, try to ping it (dot 10).
If that works, move computer to slave. Try to ping main unit (dot 1). Then try internet.
If not, can't think why. There are articles in Community on this. Search for "second router".
And on internet.
One option is to connect to WAN port on slave.