The connect engine needs to add a socket connection. I know that in the past it was not possible to imagine this scenario, where two machines would be able to connect to two or more MariaDB instances via a socket, but now I do it all the time and is part of powerful business applications. It is possible with containers, when several containers share a common directory that also holds the socket for each individual MariaDB instance. The Connect engine suffers from a lack of socket connectivity and access to its superior performance. So please add
connection='mysql://user:xxxxx@/dir/socket.sock'