Seems like you have 2 options:
Option 1: Route traffic between LAN & WLAN
This keeps the existing setup you have now pretty much and just routes traffic between LAN & WLAN as required.
First, enable IPv4 forwarding.
Second, configure iptables to permit traffic between the networks, which would look something like this:
sudo iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT
sudo iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT
The iptables change isn't persistent, but the link mentions a quick way to fix that. Alternatively you could just add these lines to your bashrc or something if you prefer.
Option 2: Bridge both interfaces together
Basically this would change the setup you have now from:
eth0 --> LAN interface, 10.42.0.0/25 subnet
wlan0 --> WLAN interface, 10.42.1.0/25 subnet
to something like this
br0 --> LAN+WLAN interface, single subnet (can be 10.42.0.0/25 or 10.42.1.0/25)
To do this you would have to disable the DHCP server & remove the IP addresses associated with wlan0 and eth0 on the debian server, create a new bridge interface, and add them to it.
You could do that with these commands:
#Install bridge utils
sudo apt-get install bridge-utils
#Create the bridge & add the interfaces
sudo brctl addbr br0
sudo brctl addif br0 eth0
sudo brctl addif br0 wlan0
sudo brctl show
#Set the IP on br0 (ex. 10.42.0.1/25) and turn up the link
sudo ip addr add 10.42.0.1/25 dev br0
sudo ip link set dev br0 up
Then, reactivate one of your DHCP servers (whichever is handling the subnet you decided to keep) on the br0 interface instead of what it was on before.
This would be similar to how most home Wi-Fi routers work. Keep in mind that this is bridge isn't persistent and will disappear after a reboot - you would need to create/edit the respective configuration files for netplan/network manager/whatever is handling networking on your server for it to survive a reboot. General instructions can be found here.
