I recently found myself in a similar situation with my second B3. This one was behind a modem/router/wifi-thingy as well (although 4G/LTE rather than ADSL).
I found the easiest way was to completely all built-in functionality of the modem/router and let the B3 take care of everything.
This was a few months ago, so steps might not be in order (and I made mistakes and had to restart), but the general idea;
1. Set up the B3 as a router/server/firewall
2. In the WAN settings of the B3, give it a static IP address, in the same range as the ADSL modem (192.168.0.2 or something).
2.1. Gateway, point it to the ADSL modem (192.168.0.1)
2.2. Default DNS, point it to the B3 itself (192.168.0.2)
3. In the ADSL modem, turn of DNS forwarding, DHCP, WiFi, Firewall, but enable all traffic to be forwarded without interference to B3 (192.168.0.2)
4. In the B3 network settings, under LAN, make sure that you enable DNS and DHCP service
5. Configure the B3 firewall
If all is set up correctly, you should need no customized hosts file on the clients. The B3 will bridge between it's own Ethernet and WiFi LAN.
All clients (ethernet and WiFi) will have 192.168.10.x addresses.
Please be aware that it is easy to make mistakes. You said you have backed up your B3 settings, but have you tested to restore them?
Can you restore them if you are unable to reach B3 through the web interface? Can you restore the ADSL modem?
- Make a clonezilla backup of /dev/sda1 on your b3 (I know, it requires pulling the disk out of the B3)
- At least make sure that you have a B3 reinstall/rescue stick prepared (and tested).
- That you have a spare unit you can use as DHCP server for B3 in rescue mode (some old router or such)
- That you can configure one of your computers to have a static IP address - the same that the B3 would have (192.168.0.2 in the example above) - so you can use it to connect to the ADSL router and reconfigure it if something goes wrong, rather than having to do a factory reset.
In short - be careful!
(And please do not hold me responsible if anything goes wrong