MariaDB error 2003: Can't connect to MySQL server on x.x.x.x

[Joe Harrington]

Server and client are each on machines connected to different local 
networks.
gnucash version 2.6.18 (3a9fbb8) and MariaDB 10.1.26 (xampp 
v.7.1.9-0-VC14) all machines running Windows 10

trying to open db using external address of server network.

server is bound to 0.0.0.0

user has full privilege with ip address of x.x.x.x

where x.x.x.x is local ip address of client machine and
external address of client network and
local address of gateway on server network

gateway on server network is configured to forward port 3306 to ip of 
server on local network.

I tried all combinations that made any sense to me and all return same 
error.

I CAN open db running gnucash on same machine as server using 
'localhost' and on a different machine on the same local network using 
local address of server. User has privilege with local ip of 2nd machine 
and local address of server.

What do I try next?