Is the issue that you have containers talking to one another using the internal docker network and you have them referencing each other based on the docker IP?
I believe the fixed IP, which shows up for me when i choose a custom bridge, is only used when you are giving the container an IP on you LAN and is not used as a way to set the internal docker IP for that container.
Are you familiar with Portainer? If so, I would try setting a container specific IP in the network settings for that particular container.