I'm a long time DD-WRT user too but I could never find a N150 travel router with DD-WRT support. I finally summoned up the courage to get OpenWRT working and found out it is not too bad especially if one's aspirations to just get WISP and OpenVPN working! OpenWRT now has a GUI called LuCI that makes thing much easier than using command lines though some comfort with command lines is needed for the installation.

Prep a USB stick by formatting it as ext4. Make sure you have a way to SSH to the router (I use putty on Windows) and a way to do SCP file transfers (I use WinSCP on Windows).

Using the guide at the web page http://www.loganmarchione.com/2014/1...ink-tl-mr3020/, connect to the router and install OpenWRT. If you have a 3rd party firmware installed, you can install from its admin page rather than the OEM install page. Stop after you've set up NTP. Reboot and connect over an Ethernet cable since that is the default.

Then copy the install-generic_VPN.tar file from https://dl.dropboxusercontent.com/u/...eneric_VPN.tar onto your PC and use a SCP client and your login credentials to copy it over to a location on the router that will persist through reboots. I used /etc. (Others have used wget directly from the router to download this file)

Use a SSH client to log in to the router, cd to /etc and extract the contents of the install file using the "tar xvf" command. cd to /etc/install-generic_VPN folder and read the file called install. (You may find it easier to follow the instructions by extracting a copy of your files on your PC). You need to make a few edits to files to suit you, the main ones being the credentials to access the router over WiFi and the other to provide your OpenVPN credentials. Configuring WISP aka WWAN credentials is optional.

Now with the mode switch in the AP position, use ./install to begin the installation which requires several reboots, each of which will require you to login again using SSH.

The way the firmware works is to first boot in AP mode where all it allows you to do is to connect to the router over WiFi but no Internet connection. Open a web page to the router's address and use the Networks -> Wireless tab to enter the credentials of your wireless network and click on Save&Apply. Check to see if you now have Internet access. Change the mode switch to WISP and reboot. You will now continue to have Internet access using the previous settings.

Connect to the router again using SSH and try invoking the VPN manually using the command "/etc/init.d/Auto_VPN_Init start". Hopefully, your OpenVPN tunnel will come up or you will get clues to get it working.

Change the mode switch to the 3G/4G position, reboot and see if you are now connected to the Internet over your VPN tunnel.

Once you are set up this way, only the WISP gateway needs to be set up using the GUI if you are on the move since the WLAN and the OpenVPN credentials do not need to change. No more fooling around with SSH commands!