Yet I already have a Wi-Fi network in place. So would I need to create a new one? I'm not sure I see why.
I'll lay out my understanding and thinking.
My understanding: you have an Xfinity device in bridge mode, so the wifi capability is deactivated and it is functioning solely as an Internet modem. To it you have connected an Airport Extreme router, which is wirelessly connected to 2 Airport Express network extenders.
You plan to get rid of the Airport Extreme router & 2 extenders and replace those with a mesh product as your router.
You plan to buy a mesh product (for sake of argument, let's say an Amazon Eero with 3 nodes), and plug one of those nodes into your Xfinity device (still in bridge mode, so still serving solely as a modem).
I'm saying when you do that, you'll likely have to set up a new network, though it should be fairly easy to do and I don't think you'll need a tech. You're asking me why you'll need to create a new network since you already have one.
It's been many years since I used an Apple Airport Extreme router, so if anything is different about a setup with one, I don't recall that.
Let me explain using my own system. I have a Motorola cable model and TP Link 6E router, and set this network up probably last year. The Motorola modem was pretty much 'plug and play.' The router, on the other hand, involved me either using a web browser interface on my desktop Mac to set parameters, or an iPhone app, both provided by TP Link.
In other words, my network settings, the names of my 3 frequency bands (2.4, 5 and 6-GHz), the passwords, all that I had to set using TP Link app.s. Let's say I named my home wifi network JoeBlow1.
Let's say I buy an Amazon Eero mesh network, toss my current TP Link router in the trash, plug the Eero node into my Motorola modem. Will this work?
Well, if I go get on my iMac I won't have Internet connection, and JoeBlow1 won't show up as an available wifi network. That network info. was on my TP Link router, now sitting in the trash.
So I would expect to have to use Amazon's Eero app. (whether a phone app., or web browser-based app. or similar), use the default username and password to access it, name my new Eero-based wife network, and go.
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In other words, I think your current wifi network is embedded in your Apple Airport Extreme router, that when you throw out the Airport Extreme you are effectively throwing out your old wifi network with it, so when you attach a new router to the Xfininity-in-bridge-mode, you'll need to set up your new wifi network.
Apologies if I made a mistake and there's no need for all this. I'm no network guru; I've set up home networks a few times. In a nutshell, a new router tends to mean a new network.
But...if you like your current network name and password, I suppose you could use those for your new network, in which case maybe you won't need to put in network log-in info. on each of your new devices.