There firmware rescue only available for certain models. For model RT-AC1300UHP only have firmware update. There is no utilities for downloading.

I have just bought AC1300. The firmware version is 3.0.0.4.380_7485

Notice there is an update version 3.0.0.4.380.8119 released on 30 October 2017. Should we update it? Currently I'm trying to do the setup but it shows disconnected at the internet status. After I've done set up it says incorrect username and password. I've checked many times and reset the router. The unifi username and password are correct. But it still doesn't work. Is it related to the new firmware update? Or other steps which I could have missed out?

https://www.asus.com/my/Networking/RT-AC130...pDesk_Download/

Yes. Able to connect to the ISP now.. This router needs the user to specify the type of ISP then only able to connect. It's all set now and running awesomely as expected!

By the way, there is a latest firmware for AC1300, do you recommend to update the firmware?

I've just updated the latest firmware. But I found my router isn't working as smooth as the previous firmware version. Can I downgrade it to the previous version by downloading the previous firmware and install it? Will it cause damage to the router?