@MR.S Yes, this is exactly what I don't understand.

I updated my last reply just as you answered. So this should work, right?

@MR.S No, the Switch is connected via port 2 to LAN4 of the router. Port 2 is the router and port 10 is the one the devices are connected to. Between port 2 and the devices is an old TL-SG108E switch...

@MR.S Yes, for the IoT VLAN all ports of the router are enabled.

@GaRv3 It's still an issue. Devices connected to a switch (SG3428X-M2 v1.20) at a port configured with profile "All" will not receive an IP address outside of the "Default" VLAN. Does DHCP not work...

Okay, so it was caused by an "old" TL-SG108E which was set to a static IP and messed up all the IPs of devices behind it. I set it to use DHCP and now all IP addresses have been correctly applied by...