@Virgo 

Thanks for the answer. But this seems to be a completely different problem. Omada expects the Mgmt VLAN to be 1 at first and only shows the VLAN1 interface although communication is done at the correct VLAN.

  @mwo Managment VLAN is still available and functional on Omada V5 and should be there on V4 if you are using it.  On V5, management VLAN settings for the switches are under the switch's Config tab -> VLAN Interface -> Edit the VLAN settings here -> then you should see Management VLAN settings.

Also see this FAQ How to configure Management VLAN in Omada SDN Controller (4.4.4 or above), and the latest Omada manual should be of help Omada V5.4 manual, and latest V4 manual Omada V4.4

If not changing setting on the switch prior to adding to the network, to set up the management VLAN, use the discovery tool to first adopt the switches, then change the management settings.

  @JoeSea Yes, this is how to configure a management VLAN. But, again, this is a different problem.

I have already set up a management VLAN (which is not VLAN1). It is working with several EAP and some switches. Now, I want to add a new switch. Its interface in VLAN 1 is found, its interface in the management VLAN is not found. If I disable the interface of VLAN 1, the new switch disappears in the pending list and is not found anymore. If I re-enable the interface of VLAN 1 the switch returns to the pending list. If I try to adopt the switch, it fails because communication is not possible, which is quite logic.

So, the only thing that works is to put the controller temporarily in VLAN 1, adopt the switch, set the IP and management VLAN and then move the controller back to the management VLAN. In the meantime you get a bunch of error messages because the communication to all the other devices is lost of course.

This process used to be much easier in the past. You took the stand-alone switch, set a management VLAN and IP, it was found by the controller and could be easily adopted without any problems. This still works with older switches e.g. T1500-8T in Omada 5.7.4, but it stopped working with the new ones. I tried SG3428X, SG2218 and SX3008F.

  @mwo I think I see what your are describing, I'm surprised that presetting up the switch in standalone doesn't transpire over, as I've done this about a year ago.  I think this might be were TP-Link official will say "use the Discovery Tool", I've never used it but it seems like it is meant to talk across VLANs.

It sounds like you've come to a solution for your current round of setup, and I by no means what you to further troubleshoot.  But if you have more to do, maybe a laptop on the network with the Discovery tool will help in the future.