Docker on Mac... Can't access anything on local network

Question

Running OSX 10.11.6 (for campatibility reasons with some other software that isn't compatible with anything higher, so upgrading OSX isn't a solution) and the highest version of Docker compatible with 10.11.6, which is 17.12.0-ce-mac55 (23011).

Before I continue, I should say I'm not too knowledgeable when it comes to networks, subnets, netmasks, gateways, etc...

From inside any of my containers, I cannot access any hosts on my local network (apart from the host machine). My network config looks like this on the host:

en4: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ether 00:50:b6:69:51:f3
    inet6 fe80::250:b6ff:fe69:51f3%en4 prefixlen 64 scopeid 0x4
    inet 172.25.18.19 netmask 0xfffffe00 broadcast 172.25.19.255
    nd6 options=1<PERFORMNUD>
    media: autoselect (1000baseT <full-duplex>)
    status: active  

and like this in a container:

eth0      Link encap:Ethernet  HWaddr 02:42:C0:A8:10:04
          inet addr:192.168.16.4  Bcast:192.168.31.255  Mask:255.255.240.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:24 errors:0 dropped:0 overruns:0 frame:0
          TX packets:30 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:1683 (1.6 KiB)  TX bytes:2551 (2.4 KiB)

For example, from the host, I can telnet to an HP JetDirect box on 172.25.33.51, but not from any container. Similarly, I can get a response from our Navision server's SOAP server from the host, on 172.20.38.62, but not from any container. Same story with a few other machines on slightly different IPs.

Problem is, I need to write an integration bundle that connects to Navision.

Any ideas how I can get access to hosts on the network?