I am trying to port OpenWRT on a device which as one port marked as WAN/ LAN. Other ethernet port is marked as LAN. It is based on QCA9531. The device came with a very old verion of OpenWRT installed (based on Barrier Breaker). I had worked on porting another device to Openwrt-19.07.7. So I am working on that code base. But any help in this regard is welcome.
Did I understand correctly that you are trying to port your new device to OpenWrt-19.07.7?
For new devices, you should always start with the latest version in master. Take one of the other QCA9531-based devices as template and modify it accordingly: https://openwrt.org/tag/qca9531?do=showtag&tag=QCA9531 Make sure to pick the ath79 target, as ar71xx is removed in current master.
What help are you requesting? Your subject line is kind of vague, so it's not clear what the issue is, except that you are running a very old version of OpenWrt.
What is the output of:
ubus call system board