Dear @TL-ER5120-user, 

Layer 3 is configured correctly and fully working, but I'm not able to connect the TL-ER5120.

 

Did you configure Multi-Nets NAT and Static Routing on the TL-ER5120 router?

When our SMB router forwarding the data to the internet, the router will check the source IP address of this packet firstly. If this IP address belongs to different subnets from the LAN IP address of the router, the router will drop this packet.

How to solve this problem? Configure Multi-Nets NAT on the router, and we only need to add these subnets on Multi-Nets NAT, the router will transfer these packets to the internet. If you only configure Multi-Nets NAT, the packet can forward to the internet, but the replied packet cannot send to the hosts. The hosts belonging to the different subnets are different from the LAN IP address of the router, we need to increase the route entries to these subnets on the router, so we need to configure static routing as well.

The TL-ER5120 is configured with static address 192.168.0.254 and DHCP-Server turned off

while Layer 3 is having IP 192.168.0.1 static configured for inter VLAN routing. 

The gateway of your network should still be the ER5120's LAN IP, which is 192.168.0.254, though its DHCP Server is turned off.

For your reference, please refer to the configuration example below and see if it helps.

How to build up a multi-nets network via Multi-Nets NAT feature on TP-Link router with L2+/L3 switches.