Not sure if this is a good idea, but here goes.
Leave your hub in router mode with WiFi turned off, this will enable your device to keep working on 192.168.0.xx
Connect the 68U to the hub LAN to WAN, so the cable is in the 68U's WAN port, the 68U's DHCP is enabled and its IP address is 192.168.x.yy (where x is not 0). As you will now have two LAN IP segments, devices on one network will not be able to communicate with the other network.
You can subsequently connect the Synology to the 68U's network. The Synology will need a fixed IP address, I have set mine within the DHCP range of my router. I also port forwarded TCP ports 5000 (for http) and 5001 (for https). So, I have the option to access my Synology externally like this:
http://<my external IP address>:5000
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Hub 3.0, TP-Link Archer C8, TP-Link TL-SG1008D 8-port gigabit switch, V6