How to access from second ethernet port to home network on the first ethernet port

Glad this helps.  If you manually put in a 192.168.12.x address in your Asus router instead of a randomly assigned address from T-Mobile gateway, you can put in a persistent option in your laptop route statement to have this route up after all restarts.  I recommend 192.168.12.254, T-Mobile should not assign this address to any other device.

@AngelaK Thanks, that works and it's safer than my alternative.  Your approach allows for all of my devices to stay behind the router while only the desktop exposing to the internet.  My alternative approach that's also working is setting my Asus to be the "Access Point".  This way, my Asus and all devices connected to it are on 192.168.12.x subnet.  However, this causes Asus to turn off all Firewall protection.

@copz1998 The Arcadyan KVD21 has two ethernet out ports.  Let's say that I connect my desktop-pc to port 1 and laptop to port 2, then both devices will see default gateway as 192.168.12.1.  However, I connect my Asus Wireless router to one of the port and the desktop-pc to the other.  My setup ended up being:

T-Mobile gateway -->Asus Router-->Other Devices….These devices will print out 192.168.50.1 as default gateway because that’s coming from the Asus Router.

T-Mobile gateway -->Desktop….This one will print out 192.168.12.1 as default gateway because it’s coming directly from the Arcadyan KVD21.

@DemDemD, ok. Because your gateway is connected to the T-Mobile internet backbone, ip # of the gateway should be sufficient. For troubleshooting: 1) make sure your Asus router is setup as DHCP to automatically connect to the T-Mobile gateway (Arcadian) internet access. 2) do the same with your desktop and make sure the network DNS settings are DHCP (automatic). However, manually entering well known DNS settings also works: 8.8.8.8 and 1.1.1.1 

It looks like your home network is setup like this: T-Mobile gateway > Asus router > desktop and other devices

Let us know how it works. 

@copz1998, I put 192.168.12.1 in the DNS setting properties of the IPV4 of my desktop ethernet network but it doesn't work.  Please note that the Default Gateway of the desktop-pc is showing as 192.168.12.1 while the Default Gateway of the Asus Wireless Router is showing as 192.168.50.1.

@DemDemD, ok. Then you need to plug your ethernet cable into the 2nd port of the gateway from your desktop, then add the following to your DNS settings; 192.168.12.1 (the IP # of the gateway) to your desktop’s network settings.

OK? 

@copz1998 , I understand that if I plug my desktop ethernet cable into my wireless router then I would get access to the wireless router's network.  However, I don't want to do that.  I want to use the second port available from the T-Mobile receiver as this will provide a much faster connection (being direct) rather than going through the wireless router.  I figured since my wireless router and my desktop are both connected directly to the receiver that there may be a way for them to see each other.

@DemDemD, plug your desktop ethernet cable into your wireless router. The premise is - all devices on your network should get Internet access via your wifi router. the wifi router gets its Internet access from the T-Mobile gateway. 

Let us know if it works for you.