I am currently working on Raspberry Pi 3B+ (Stretch 9) and I have been stumped on how to be connected online wirelessly while also having my LAN port configured to receive and send data via a crossover Ethernet cable to an external piece of hardware (Note: the hardware via the LAN port is being interrogated with a Java program and **does not** need a bridged connection). The hardware defaults to the following settings:

    IP Address: 192.168.1.100
    Subnet Mask: 255.255.255.0
    Gateway: 192.168.1.1

I can make the hardware work by the edit shown below but it disables my wireless connection.

Editing the /etc/dhcpcd.conf with:

    interface eth0
    static ip_address=192.168.1.200/24
    static routers=192.168.1.1
    static domain_name_servers=192.168.1.1

If this might be any help, here is the output of route -n:

    Kernel IP routing table
            Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
            0.0.0.0         192.168.1.1     0.0.0.0         UG    202    0        0 eth0
            0.0.0.0         192.168.1.8     0.0.0.0         UG    303    0        0 wlan0
            192.168.1.0     0.0.0.0         255.255.255.0   U     202    0        0 eth0
            192.168.1.0     0.0.0.0         255.255.255.0   U     303    0        0 wlan0