Simple router using Debian and Firewall-cmd as a base
Here is a quick rundown on how to make a simple Debian box into a router Removed un-needed items
apt purge iptables
Install required items
apt install bridge-utils firewalld dnsmasq
Enable IP Forwarding
sed -i 's/#net.ipv4.ip_forward=1/net.ipv4.ip_forward=1/g' /etc/sysctl.conf
Find NICs
ip a to find the NICs
Now we create the bridge. This is assuming your device has three NICs and you want two for LAN and one for WAN
nano /etc/network/interfaces