Here comes a post to improve that great router from TP-LINK.
Of course, I suppose you’re running openwrt on it to use the whole power of it.
The 1st I saw was that you have available a plenty set of packages… but you don’t have enough ROM to install them :-P
Inspecting a bit, I saw that under /tmp you have a “lot” of free space! (about 14MB), that will be enough to allow run openvpn without problems.
The main problem? Its a ramdisk, so each boot, everything there disappears.
What I’ve done? Install the basics with opkg and wget on boot the rest ;-)