Dear @Xstreem,

with my previous L2 switch (Netgear JGS524PE) I was able anyway to assign multiple ports to multiple VLANs without having a VLAN aware gateway.

Sorry that I didn't make it clear in my last reply. Apparently, I mixed your question with this post, I should have answered that when you configure multiple SSIDs to multiple VLANs on the EAP for Internet access, then a VLAN aware gateway is required (the gateway needs to forward the tagged packets back to the EAP since communication is bidirectional). 

So for example I was able to create VLAN 10 and VLAN 20 and put PORT1 where router non VLAN Aware is connected on both.

Then assign PORT 2 = PC-A on VLAN 10  and  PORT 3 = PC-B on VLAN 20

In my view, this works because the PCs can communicate with the router in the default VLAN since they are all no VLAN Aware.

I'm not sure how to implement this thing in Omada with TL-SG2428P switch

Based on the example you gave above, the configuration in the Omada Controller could be like this:

1)  Wired Networks --> LAN --> Networks, create two new LAN to add VLAN10 and VLAN20 respectively.

ie, Name: V10, VLAN: 10;

     Name V20, VLAN: 20.

2) On the Profile tab, create port profiles to define how the packets in both ingress and egress directions are handled:

ie, Port profile name: PCA, Native Network: V10 (PVID=10), Untagged Network: V10, LAN (VLAN10, VLAN1)

    Port profile name: PCB, Native Network: V20 (PVID=20), Untagged Network: V20, LAN (VLAN20, VLAN1)

    Port profile name: Router, Native Network: LAN (PVID=1), Untagged Network: V10, V20, LAN (VLAN10, VLAN20, VLAN1)

3) On the Switch Settings tab, edit the switch TL-SG2428P and go to Ports to assign the above port profile to the desired ports of the switch to activate the configurations. So edit Port1, Port2, Port3, and select Profile as Router, PCA, PCB respectively.

4) Connect PC-A to Port2, PC-B to Port3 and Router to Port1 on the switch TL-SG2428P, then PC-A and PC-B cannot communicate with each other, but both of them can communicate with the Router.

Hope the simple configuration example above could help you with the VLAN configuration in the Omada Controller. 

As mentioned, you could also check the configuration example given in the Controller to understand how to configure wired LAN network.