The device firmware version? what's the network topology? Do you have some specific access rules or policy routing, or something else??

Need to enable remote management feature. Refer to this material: https://www.tp-link.com/en/faq-2135.html.

Is your network topology as PC0----------[WAN]TL-ER6120[LAN]----------PCx ? And you just want to establish RDP connection from PC0 to PCx, right? If it is really the case, you just need to do port...

access control would be helpful

And how many clients are you coming with

Your device should be TL-ER604W V2 160825. And you should report what feature you're turning on? Is reset helpful?