All the stuff out there is written about bridges is also about making a wifi hotspot. Most of it is wrong for old /etc/network times. Jessie and before. I don’t want a hotspot. Very little about bridging two physical connections and dhcpcd.
I have 4g dongle. Plugged into the raspberry pi and plays and reaches the internet. It is wwan0 whose IP is now 192.168.9.100, the dongle gateway is 192.168.9.1, the dongle provides the IP addresses DNS. I want to bridge wwan0 to eth0 so eth0 devices can reach the internet. I have been using brctl (from bridge-utils). I prefer to bridge, I do not want to NAT again. I don’t need any firewall here. So any help in bridging two physical connections much appreciated by me and many others.