mthdcs-3604
This is the name that your WIU got at the factory. Consider it just like the name that a PS2 or PS# engine comes with. The difference is, however, that you can actually change this name via LuCI,
192.168.1.76
This is the IP address that your router assigned because you connected via Ethernet. The address that MTH provides is only for when the WIU acts as its own router. Since you were trying to connect via your home network rather than directly to the WIU wirelessly, the MTH-provided IP address couldn't work.
This is the way that networking works in general and MTH is following the "rules" by doing things the way that they do.
Note that this IP address is subject to change every time you turn off the WIU and/or the router, since your router will a sign the first available IP address it has each time it sees the WIU after both devices have been powered on. Depending on what other devices its sees first, the WIU's IP address may be different.
If you can learn how to configure your router yourself, you can assign the WIU a unique, repeatable IP address so that subsequent firmware updates can be better facilitated.
