I ended up getting it to work by enabling "Host access to custom networks" in the Docker System Settings, advanced view. Without that it won't allow Dockers to talk to each other, it seems.
In the phpipam docker, it must be set to the br0 Network Type. Then set a Fixed IP address and set IPAM_DATABASE_WEBHOST to the same address. IPAM_DATABASE_HOST should be set to the IP address of MariaDB, mine is the same as the Unraid server IP address.
I ran the new install with using the automatic database creation and it worked without a hitch. MariaDB is set to Network Type - Bridge.