There are several things you need pay attention to:
1. The original antennas of the TP-Link products comply with local regulations. Using some higher gain antennas, especially for outdoor products, may violate local law. Therefore, please consult the local authorities before you buy antennas.
2. High gain out-door directional antenna is usually used for long distance point-to-point transmission, while omni-directional antenna is for covering an area.
3. The connector type of the antenna. The connector type of TP-Link routers and APs is RP-SMA. Make sure your antenna has RP-SMA connector.
4. The impedance of the antenna. The impedance should be 50 Ohms. Impedance mismatch will not improve the signal quality, or even make the signal strength worse. Consult the vendor if you are not sure about it.
5. The frequency of the antenna. Make sure you choose antenna frequency according to the working frequency of the products.
6. For dual-band routers such as TL-WDR4300, TL-WDR3600 and TL-WDR3500 you need to replace the antennas with dual-band ones. Currently TP-Link does not provide such antennas.
7. For routers with more than 1 antenna, it is recommended to replace them all.
By the way, you can check the price of TP-Link router.
Learn more:
Why Can't I Log In To The Web Management Utility Of The TP-Link Device?
What Is Running Out of IPv4? What Is IPv6?
What Can I Do If The Wireless Network Is Unstable Via My Wireless Router/Modem Router?
What Can I Do If I Forget The Login Password Of TP-Link Wireless Router?
How to Change the MTU Settings in TP-Link WiFi Routers?
FAQs About The TP-Link Routers' speed
How To Set Up IPv6 Service On The TP-Link Wireless Router?