Did not see this function on its datasheet. I think it doesn't support this function. You may need to change other model if you must need this function.

If you need to manage your router from LAN, just access 192.168.0.1. If you need to access from WAN, you need to set up remote management. TP-Link should has the FAQ for this function.

@BigD You can take a cable from the switch and plug into the WAN on the TL-ER5120, but why you still want to take a cable from the switch and plug into the LAN on the ER5120? I think you can remove...

@PeterS Do you mean that you can access other devices even though your client doesn't pass the authentication? I think you may contact tp-link support for help.

@Carles You should make sure what IP your switch is using. Then check if you can ping the IP address. Then try to telnet the switch. If you can telnet, still can configure the switch through CLI...

@hughLink T2600G-28TS v3. You can separate two wifi network on one AP. But need your AP supports multi-SSID and VLAN. You can refer to the FAQ. https://www.tp-link.com/en/support/faq/1849/