@Guo Hello,
RE300 has higher spec than RE210 on 5Ghz and its overall wireless performance should be better. (RE300 5 GHz Link speed :Up to 867 Mbps vs RE200 5Ghz link speed: 5GHz:Up to 433Mbps)
For ' RE300 5G light is blink ' do you mean the 5Ghz Led goes off from time to time or it flashes in a regular way?
Make sure router's 5Ghz signal is strong enough at RE300's location, and you could locate router closer or adjust the RE's location as the guide below:
How to place your wireless Range Extender for optimal reception and performance?
https://community.tp-link.com/en/home/forum/topic/269204
Please also try a different Wi-Fi channel for router's 5Ghz, Channel 36/40/44/48 is recommended.