tl;dr This is a guide which steps through installing OpenWRT to TP-Link Archer C7
You will need
TP-Link Archer C7 AC1750 Ver:2.0: http://www.tp-link.com/en/products/details/cat-9_Archer-C7.html
Direct firmware download link: https://downloads.openwrt.org/latest/ar71xx/generic/openwrt-15.05.1-ar71xx-generic-archer-c7-v2-squashfs-factory.bin
UPDATE Nov 17:
There seems to be a bug in ath10k kernel module causing connection drops becuse of no ack received from wireless station (eg. your laptop) to AP (the router).
Mar 27 08:13:56 xxxxxx daemon.info hostapd: wlan0: STA xx:xx:xx:xx:xx:xx IEEE 802.11: disconnected due to excessive missing ACKs
Ack's are sent correctly but bug in ath10k causes them to disappear and low_ack is fired causing station auto-disassociation. Workaround for this is to disable 'disassoc_low_ack' feature (with a value of '0') to /etc/config/wireless under desired wifi-iface configuration.
# cat /etc/config/wireless config wifi-iface option device 'radio0' option network 'lan' option mode 'ap' option disassoc_low_ack '0' ...
Labels at box
Labels telling the SN and HW revision. Check that the revision is:
The unit itself and the backpanel
Stock firmware login
Connect unit's LAN1 (yellow) port to your computer with a RJ45 cable. Do not use a static ip address at your computer, use DHCP instead. Power up the device, wait 60secs and then navigate to http://192.168.0.1/ and login with admin:admin
Stock firmware update
Navigate to System Tools -> Firmware Upgrade and select downloaded *-factory.bin file. Then click Upgrade.
After the unit's reboot you need to unplug and then re-plug RJ45 cable, this causes the DHCP to renew the ip address which is now changed to http://192.168.1.1/ Now navigate there and login with root:root
In fact OpenWRT is designed to be used through a command line interface but someone has developed a very decent web interface called LuCI.
OpenWRT LuCI web interface tour https://www.youtube.com/watch?v=kLLtZH2Bc0w