Hi
Parpin
,
You got me curious, so I pulled out an old 2925n (older than the 2866) and started configuring it to do what you want, my only real question is, what is your “…primary internet router…”, is it a DrayTek router?
Either way, this should still work; using the 2866 as an 'Access Point':
Go to ‘WAN >> General Setup’ and disable all WANs, then go to ‘LAN >> General Setup’ and configure LAN1 by first disabling the DHCP Server option and setting the IP of the 2866 router ('
Network Configuration' section) to a static IP and mask somewhere in the range of your “Primary” router’s
main LAN subnet (the non guest LAN), tick the ‘Enable’ box and click ‘OK’; it will most likely ask to reboot, so 'OK' that and reboot.
Log back in to the 2866 at its new LAN1 IP you just gave it, and go to ‘LAN >> General Setup’ again but this time click on the settings for LAN2 instead. Same again, disable the DHCP Server option and set the 2866 router ('
Network Configuration' section) to a static IP and mask somewhere in the range of your “Primary” router’s
guest LAN subnet, ‘Enable’ that too and click ‘OK’ and it should ask to reboot again.
Log in again, and now go to the ‘LAN >> VLAN Configuration’ page, ‘Enable’ it, and tick all the VLAN0 P1-P5 tick boxes then
only tick the SSID1 of 2.4GHz and SSID1 of 5GHz boxes, set the ‘Subnet’ to LAN1 and
do not tick the VLAN ID tick box, leave it un-ticked.
Now on VLAN1, tick only the P1 tick box and also SSID2 of 2.4GHz and SSID2 of 5GHz tick boxes, set the ‘Subnet’ to LAN2 and this time tick the VLAN ID box and enter the ID TAG for your guest LAN used on your “Primary” router. Now ‘OK’ this page; and it may ask to reboot again. (
Note: If it warns you about ticking any un-ticked Port boxes on VLAN0, then you'll need to have them all ticked to enable the VLAN functionality, so do that and click 'OK' again)
You can now configure your 'Wireless LAN >> General Setup' SSID1 & SSID2 (2.4GHz & 5GHz respectively) to match how you have it set on your other APs, minus the VLAN IDs as these are already setup, as above.
All that’s left to do now is to configure the ‘Primary’ router, or your managed switch attached to your ‘Primary’ router, so that you can connect
Port 1 of the 2866 to that device’s VLAN configured port.
If it’s another DrayTek router, then in the ‘LAN >> VLAN Configuration’ page, tick whichever port will be connected to the 2866’s Port 1 on the VLAN of your main LAN, if you don’t have a VLAN ID set for that VLAN, then great, you only need to tick the port you’re connecting to the 2866 and set the 'Subnet' to the correct one if it isn't already set correctly. For the next VLAN, the guest LAN VLAN, tick the same port that connects to the 2866 and hopefully this VLAN
does have an ID TAG set, which should match the one you’ve setup already on the 2866 for VLAN1 LAN2, and also the correct 'Subnet' that is your guest LAN subnet.
(Note, if your main LAN
does have a VLAN ID TAG set against it, then you’ll need to go back to the 2866 and enable the ID TAG option and enter that same matching ID number there too.)
If it’s a managed switch your 2866 is connecting to, then setup the switch port as an ‘Access Port’ assigning your main LAN VLAN to it, and also add a Tagged ID to it of your guest VLAN as well; sometimes causing the port to become a "Hybrid" port rather than an 'Access Port', but you get the idea, un-tagged = main LAN, tagged = guest LAN.
With all that done, your 2866 should now be a working AP like the rest you have in-play.
I have just tested this using a 2862ac connected directly to the 2925n, using both the Tagged and Un-Tagged methods, tested separately, and both work fine with the main LAN VLAN. 
