@Virgo HI Virgo,

thank you for your answer.

The router configuration is:

WAN (on port 11) -> Static IP from provider, no issues

LAN:

2 groups:

Group1 (port 2-3-4): 192.168.1.0/24 lan -> vlan tag -> DHCP issues on some devices

Group2 (port 5-6-7-8-9): 10.100.0.0/22 lan -> vlan tag -> DHCP issues on some devices

The router is a new install, but with previous router (Zyxell) every device worked.

The windows PCs get DHCP and connect to the internet very good.

The secondary router / ap (Linksys, Ubiquiti and Zyxell tested) don't get the DHCP (the fall on no ip at all or 169.x class)  and also if manually set with static address don't connect to the internet. Vlan issue? But why? ?_?

Thank you again and best wishes

  @Virgo 

Hi Virgo, thanks again, you pointed the right direction but... 

I have 2 LAN: 

LAN1 -> 192.168.1.x/24 -> VLAN 1

LAN2 -> 10.100.0.x/22 -> VLAN 10

I read the document, and changed the port 7 - the one I'm using for tests:

from: TAG - PVID 10 

to: UNTAG - PVID 10

What happened? 

Windows Laptop: 

get the DHCP IP - navigate correctly - ok as it was previously 

The Linksys AP: 

get the DHCP IP from ER8411 (urray! that's a change! It didn't before) - doesn't resolves DNS (like in ping www.google.it) - doesn't connect to internet :-( 

The same windows laptop, connected to the linksys AP: gets the Linksys DHCP address (on another subnet: 192.168.4.x/24), gets the DNS passed by the TP-LINK WAN (OpenDNS) - doesn't ping - doesn't connect. 

So... a small improvement but still no connection and I can't get why... 

The laptop, if connected to the same port 7 (no switches in between) gets a 10.100.2.x address, same DNS and navigate. It also does on another port (with TAG - PVID 10), so the change from "TAG" to "UNTAG" is that the Linksys is able to get the DHCP assignation, while before it wasn't. Windows always get the DHCP (both TAG and UNTAG the port is). 

Thanks again for help and support, 

Best Whishes,