@Colin_R 

I am assuming you run Deco in its default router mode, and there is no other router in your home network between Main Deco and Internet.

RDC (Remote Desktop Client) uses RDP (Remote Desktop Protocol), so I will refer to it instead of RDC. You can get more info on it if you google for RDP instead of RDC.

To have RDP access to your hobby computer, three things are necessary:

1. You need to know public IP address of your home network;

2. You need to know IP address of your hobby computer, and it is strongly recommended it always stays the same, which means hobby computer should be set with fixed IP address;

3. You need to know which port(s) RDP uses.

When you know that, you can set port forwarding through Deco app for the port(s) used by RDP, to your hobby computer. Next time you RDP to public IP address of your home network, that connection will be redirected (forwarded) by Deco to hobby computer.

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You can find public IP address of you home network by opening web site www.whatismyip.com in browser on any device (laptop, desktop, tablet, smartphone, etc.) connected to your home network.

Note that this IP address may change from time to time, you'll need to re-check it especially if RDP access that worked before suddenly stopped working.

This FAQ article will explain how to set fixed IP address for your hobby computer https://www.tp-link.com/us/support/faq/1795/

Google "What's the port number for Remote Desktop Protocol?" to find ports RDP uses.

This FAQ article will explain ho to set port forwarding in Deco, follow it after you set your hobby computer with fixed IP address: https://www.tp-link.com/us/support/faq/1797/

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A word of warning. Opening RDP access to your hobby computer creates serious security risk for it and for the rest your home network. RDP is fairy popular protocol and hackers are known to abuse it to gain access to computers.

To get more info on that topic, google "RDP exploits"

You may decide not to open RDP access after you read about RDP exploits, or you may find recommendations to mitigate risk. If possible, follow them.

Once again, you are opening remote access to your hobby computer at your own risk.