I applied the Chaos Calmer OpenWRT firmware upgrade on the TP Link WDR4300 router. Very important to note that if you already use OpenWRT on the router, install the sysupgrade and not the factory binary firmware file. Use factory only when upgrading from the factory TP Link firmware.
For first time install use this file https://downloads.openwrt.org/chaos_calmer/15.05/ar71xx/generic/openwrt-15.05-ar71xx-generic-tl-wdr4300-v1-squashfs-factory.bin
For upgrade from Barrier Breaker to Chaos Calmer, use this file https://downloads.openwrt.org/chaos_calmer/15.05/ar71xx/generic/openwrt-15.05-ar71xx-generic-tl-wdr4300-v1-squashfs-sysupgrade.bin
Note that ar71xx is the chip.
Now, after applying the upgrade, I could no longer use LuCi, the web interface, to login to the router using https or http. After googling around, I came to this link https://samhobbs.co.uk/2014/12/cant-connect-luci-after-openwrt-upgrade-solution, where the issue appears to be that when you upgrade, OpenWRT does not install the packages you installed on your own in barrier breaker. These have to be installed again in chaos calmer manually.
To overcome this issue open a terminal window in Linux, ssh into the router and install the missing packages as illustrated below:
Enter the root password of your OpenWRT installation. It would be the same as you used when you logged in via LuCi.
opkg update <enter>
This command updates the packages list. It is just like apt-get in Ubuntu or yum in Fedora.
opkg install uhttpd-mod-tls <enter>
/etc/init.d/uhttpd restart <enter>
Now login via web interface will work. The issue was the missing uhttpd-mod-tls package.