@Marc_NL hello Marc, I just bought the same router and despite many other difficulties this might be quite simple.

Go in Wired Networks -> LAN -> Create New LAN

Then choose interface and check the box LAN

Assign a VLAN ID, for example 40

Assign a gateway like: 192.168.40.1/24

Then press update DHCP range or set your range manually in DHCP Range

Then go in profile, create one, set native network with the interface just created

choose tagged/untagged networks

Assign this profile to a port

A client with DHCP on will get an IP in the range specified

That's it

Hope this helps

@Xstreem thanks! This helps a lot!

Due to the nature of my business I cannot use cloud-based software, but I discovered that there is a local variant too! The downside is, that the web gui is not available anymore and my three switches does not appear as switches (could be logical as they are not Omada-complient), but they are not even there as client... Had to use an ip scanner to get their IP's 

But back to your solution; I just tried it quickly before office hours and it easy until the step: "Assign this profile to a port" I get confused. I don't see any ports (or do you mean the ports after "Interface?) to assign them to in any of the menu choices. What is it that I overlook?

I moved the setup to a clean windows machine with Omada and Angry scanner en no connection to anything else, then to the router; no internet either.

First I did a firmware reset on the router, installed Omada and it went smooth, until I 

Adopting the router in the local installation of Omada was swift and painless and after the initial settings I got to step where it says to make the profile and then nothing. No device present and adoption was not possible anymore. Only a factory reset and a complete reinstall of the software could bring it back to life, but only after about an hour of trying to adopt.

When connected, I've added the VLAN as described and in "Profile" there is an entry with the same name as the VLAN I just made. It is not possible to edit this, only to view the settings. Still there are no ports available in "Switch settings", or anywhere else.

How can I make them ports (1-5) visible?

@Marc_NL is your switch supported by Omada? There are very few of them at the moment. If it is not a supported model you will have to use the devices in their own gui separately. 

@Xstreem yes it is, it is printed on the box and says so in de leeflet.

The switches I have aren't and they don't even show up in Omada.

@Marc_NL what is the model? The profile created with the interface you can delete and make a new which is possible to edit. Then go to devices click on the switch, then ports and in each port you can assign a profile

The router is a TL-R605 and the switches are TL-SG1016DE

The router is the only device in the device list and the ports on this device doesn't show up anywhere either. There are no ports available; "No entry in the table" says Omada and I can't add any either

@Xstreem The switch is in the client list of the router now, but there are no ports anywhere to be found in Omada

@Marc_NL marc that switch is not supported by Omada see here https://www.tp-link.com/us/omada_compatibility_list/ so that's why you don't see in the devices list and therefore you cannot apply profiles etc...

if you also che the datsheet here https://static.tp-link.com/2020/202001/20200115/TL-SG1016DE(UN)%204.0&TL-SG1024DE(UN)%204.0.pdf

you can see that management is via GUI or Easy Smart Configuration Utility

@Xstreem That is correct; the switches are not supported by Omada.

But the 4 lan-ports on the router itself are... I hope... and they are not in the list either or anywhere else as far as I can see