Can I create a second wifi network using an airport extreme to share files and stream videos and music?

With the correct set up, you should be able to connect an AirPort Extreme to your current mesh router and use the current Wi-Fi mesh network to access a hard drive connected to the AirPort Extreme. So, everything will be on the same network.

A second Wi-Fi network should not be needed.

Want to give it a try? If yes, we need to know whether you will be using a Mac (preferable) or an iPhone / iPad to set up the AirPort Extreme.

If this works, you won't need the other 2 AirPort Extremes at all.

I think TP-Link likely misunderstood what you were asking, but I don' t have a TP-Link router here to test.

You won't be connecting the AirPort Extreme to the TP-Link using USB. You will be connecting the AirPort Extreme to the TP-Link router using an Ethernet cable. The hard drive will remain connected to the USB port on the AirPort Extreme.

My neighbor has an Eero mesh system that he bought about a year ago. He wanted to use his AirPort Extreme with a hard drive connected to the USB port on the AirPort with the Eero system.

It took maybe 5 minutes to set this up. Shouldn't take much longer if you decide to give it a try, since we can use some setup shortcuts to test things out. You won't have to change anything on the AirPort Extreme for the test.

There is no need to create a second network and switch between them to access different devices and destinations. Your home network is data highway with multiple on-ramps and off-ramps.

Both Ethernet and Wi-Fi packets have addresses on them, and can be Switched around on and off your network. The device that controls that traffic is the Switch portion of your Router.

A simple example case is when you Print to a network-connected printer. The packets moving toward the Printer have a local address on them, and when they reach the Switch portion of your Router, those packets are sent, locally, back across your network toward the Printer. Your Internet connection is not involved in such packets.

Another case is when you enable Screen Sharing or File Sharing, and access the screen or files from a different computer on your connected network. The internet connection of your Router is not involved -- the packets are sent locally.

<< you can access a USB hard drive connected to it [AirPort Extreme]. >>

You can NOT continue to use an Airport extreme to hold Time Machine files or any other files.

The ability to use a drive connected to an AirPort Extreme is deprecated. Apple is retiring the Server protocol, Apple File Protocol (AFP) used to talk across the network to Time Capsule and similar devices.

Users report that if you ERASE such a drive under MacOS 26 Tahoe, the drive can NOT be used again, for anything.

MacOS 27 will have no support whatsoever for those drives. That will come as a rude awakening for users who ignored the warnings.