OpenWrt 15.05.1 installation in Pi 3 B+

Hello everyone! I am new in this community. For my study, I want to install openwrt 15.05.1 in a pi 3 B+. But I cannot find the proper image for that. Can anyone help me regarding this?

Support for RPi 3B+ was added only in 18.06, i.e. there is no 15.05.1 image available for this device.