question , can someone provide me with link to download the latest compatible version of OpenWRT for Linksys WRT1900ACS V2 ? and I need also how to configure VPN connection on this secondary router as PPTP or L2TP connection with existing PPPoE router.
Official OpenWrt packages only contain the bare minimum, plus LuCI, allowing for the user to customize their router themselves. If you'd like specific packages and custom configs included within the image, you'd benefit from compiling your own:
I also created a lite touch script that auto sets up the build environment in Ubuntu and builds the image, with you only needing to utilize the MenuConfig GUI to select the WRT1900ACS and packages you want in the image.
As to configuring a VPN, what errors were you getting (please post the error output within code boxes)?