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.
There is a limit on cable length; 100 metres.
It connects LAN to LAN.
See how this goes.
Unplug the connecting cable.
Make an ip address reservation on the main Archer for the mac address of the faulty Archer; say 192.168.1.10.
On the faulty Archer, do a factory reset, change its ip address to 192.168.1.10 and save. This disconnects your browser session so reconnect on the dot 10 address. Set the default gateway address and DNS server address to 192.168.1.1. Disable the DHCP function and save. Plug in the connecting cable.
See that you can access the faulty Archer web server, then the main Archer web server from ethernet and wifi. And the reverse. Then internet.
The DSL settings on slave shouldn't matter since the WAN port won't be used.
Having those other settings disappear might be ok. The router might learn them.
Do you have a computer you can connect using cable? Avoid wifi initially.
From main router, try to ping the slave.
From slave router, try to ping the main.
I changed to NBN recently and was able to connect my ADSL router to the new one.
On the slave, there is only change ip address and disable dhcp.
After you disconnect the slave and can access the master, is there anything in the system log that might indicate the problem?