All of these devices are in the "4/32" category and can only run OpenWrt 18.06 -- support has been dropped on all subsequent versions. This older version of OpenWrt is now obsolete, unsupported, and may contain serious security vulnerabilities.
If you decide to run OpenWrt, you can use the "Recovery Instructions" section of the device info page... but you should seriously consider getting new hardware since this version is no longer supported.
This device is NOT RECOMMENDED for future use with OpenWrt due to low flash/ram. DO NOT BUY DEVICES WITH 4MB FLASH / 32MB RAM if you intend to flash an up-to-date and secure OpenWrt version (18.06 or later) onto it! See 4/32 warning for details.
1) This device does not have sufficient resources (flash and/or RAM) to provide secure and reliable operation.
This means that even setting a password or changing simple network settings might not be possible any more, rendering the device effectively useless. See OpenWrt on 4/32 devices what you can do now.
2) OpenWrt support for this device will end after 2019.
19.07 will be the last official build for 4/32 devices. After 19.07, no further OpenWrt images will be built for 4/32 devices. See OpenWrt on 4/32 devices what you can do now.
Beyond that, these older devices do not have the CPU horsepower, RAM, or storage to handle all but very basic routing relatively slow internet connections. This device has 100Mbps ethernet ports and probably can only handle about half that in terms of actual practical routing bandwidth.
It will not be possible at all to install a VPN if you're running 18.06 or higher (even on the tiny builds) as there simply is not enough memory to do that. And, even if you were to use a very old version (say 17.01 LEDE) which would leave you more available space, these older devices cannot handle VPN at any reasonable bandwidth due to the slow CPU performance.
Why? I have read this before over a year. I am telling that its wrong to not support those devices because they work perfectly fine.
Have you tested this? Because this is not true! Full 100Mbit is working fine on a 100Mbit internet connection. The hardware revision 6 have modern 750Mhz CPU and no issues.
I also have a 8/64 device: WNR2200 . When you install latest image wnr2200 its working way worse then the modern TL-WR940N.
For Wireguard you should build your own image and drop Luci.
I dont install any additional software on mine and can use it perfectly fine with Luci installed.