I have created an image for Buffalo WZR-HP-G300NH based on OpenWRT master (ath79) branch. The device has two variants, one based on switch rtl8366s and the other is based on a switch rtl8366rb. I am calling out for people who have the latter device. I believe the image supports both variants but I cannot be sure before someone with the device verifies this.
The source code changes can be looked at github
The images can be downloaded at here. There are variants for both kernel 4.19 and 5.4 and they don't contain additional packages but luci.