How do you test the speed? Getting 54Mbps download speed is not low.

In AP Client router mode, WR743ND connects to the front router as a client, its channel and bandwidth depends on the front router.

http://forum.tp-link.com/showthread.php?78627-Tp-Link-Archer-C2&p=153579&viewfull=1#post153579
Check the post#3 by Tobacco, there is something we need to know about wireless speed.

http://kb.netgear.com/app/answers/detail/a_id/19668/~/differences-between-link-rate-and-transfer-speed-and-throughput

I did the test on Windows wifi properties window and android mobile app (it is transfer speed, not download speed). I think the reason is using 11bgn mixed mode, but there is no way to set 11n only and 40MHz channel width.

The network is secured with WPA2-PSK, AES instead of TKIP or WEP?
Since TL-WR743ND works in client router mode, there is a root wireless router, right?
What's the speed if you connect wireless clients to it bypassing WR743?

Give us screenshot from status and wireless settings tab.

The network is secured with WPA2-PSK, AES instead of TKIP or WEP?
- it is WPA2-PSK, AES
Since TL-WR743ND works in client router mode, there is a root wireless router, right?
- yes
What's the speed if you connect wireless clients to it bypassing WR743?
- also 54Mbps

Screenshot of wifi settings on my router (connected to parent one) and wifi status window on Windows - http://postimg.org/image/be2qa5zkx/

Your main router?

It is ASUS WL-520gC, but I think it doesnt matter. What I only need to know is if and how to set mode to 11n only and channel width to 40MHz on TP-LINK router.