I was trying to connect to my old DIR827, as a separate network, with IP (for example) 192.168.2.0.
The firmware does not include that option. You can only set a static route on the WAN side, but not on the LAN side.
(if you look in the ADVANCED > ROUTING page, on the right there's the "Interface" column, with a select box that contains only 1 option, WAN. If you look at the page source, that box has <option value="WAN-1">WAN ([YOUR current WAN IP])</option>
There is no other option allowed in the firmware, so no way to choose LAN instead of WAN.
Nothing that the guys at customer support can do about that...