A factory reset is never a good thing, all it does is erase data that might help Sonos help you.

If I’m not mistaken, before you can add the Era 100s as surrounds, you first need to add them as separate rooms in the Sonos controller. Once you’ve done that, you should be able to add them as surrounds to your Arc.  Note, however, the Era series of speakers can not be added as surrounds to the PLAYBAR. 

Thanks Bruce. I did add them to Sonos System and separate rooms before trying to add them as surround speakers. 
 

I am trying to add Era 100s as surround to Sonos Arc. Based on my reading, Era 100s can be added as surround to Arc.

Just wanted to be sure. Have you added a sub? I’m wondering …oh, damn. You haven’t turned off the WiFi in your Arc because it’s wired, have you? That would stop the surrounds from connecting to it. Hmm. It wouldn’t show the Ones if you had. Unless the Ones are wired currently. I’m also wondering if your router is blocking proxied IP addresses. See if you can (temporarily) set up the Ones as surrounds. If they work, and the Era 100s won’t, I’d call Sonos Support directly to discuss it. It shouldn’t be that way. 

Thanks Bruce! I will call Sonos support 

@ASingh 

Maybe you also use a wifi repeater with your router?

That also could cause this behavior if the Eras are connected to the repeater. 

I do use Amazon Eero wifi extenders. Could that cause an issue? If yes, what's the best way the resolve it. Thanks 

Are the Era 100s in a stereo pair? You cannot add them as surrounds when they are in that configuration. If so, separate the stereo pair in the app, then try adding as surrounds again.

They are not in a stereo pair. Just individual speakers :(

If you turn off the WiFi extenders, are you able to add the surrounds then?

Is the Arc wired?.

Did you try @Airgetlam’s suggestion to see if the Ones will actually add as surrounds (just as an experiment)

Try powering off the Ones temporarily and try again to add surrounds

I will give it a try. I wonder if connectivity will be an issue. My router is in the basement and on the opposite corner of the home. I will give it a try.

@John B : I did try and I was able to add the Ones as Surround. I will try the second option you mentioned.

An even better test would be to place the Era 100s where the Ones currently are, with the Ones powered off

I am wondering if the Ones and Arc are connecting to the main eero device and the Era100s to an extender (or some variation on that theme)

Do you have an option to set up the satellites in “bridge mode” or something like that?

https://www.howtogeek.com/289542/how-to-use-the-eero-in-bridge-mode-to-keep-your-routers-advanced-features/#:~:text=In%20bridge%20mode%2C%20your%20Eeros%20act%20like%20nothing,bottom.%20Select%20%22DHCP%20%26%20NAT%22.%20Tap%20on%20%22Bridge%22.

Edit - on closer inspection this is about setting the whole Eero system up in bridge mode.  Let’s complete our initial experiments before we explore any further.

Thank you so much John. I am not at home right now. Once I go back home, I will try all these and will update the forum.

OK sounds good. It would be helpful to know whether your ISP router is still involved in your setup or the principal Eero device is now acting as your router. 

Hi everyone! Sorry for the late response. I got tied up with something else over the last few days.
 

I tired the first idea with turning off Ones and placing Era 100 in bedrooms. For some reason, it stop detecting the second Era100. I read Sonos FAQ and it asked to factory reset and add again.

I am getting the attached error after factory reset. Any idea how to  resolve this first? Can't add them as surround without a pair.

If you place the Era 100 closer to the router, does it add to your system then?

edit: Also initially when you say you were only given the option to add the Ones as surrounds, if you scrolled the picture of the One to the left, it likely would’ve brought up the Era 100s if they were showing in the system and in separate rooms.

Hi all! Thanks for your help. I was able to connect all the speakers to the system and connect Era100s as surround. 

It seems like it was a combination of software and wifi issue. I turned off 5ghz frequency on my Eero. After doing that it detected the missing Era. It asked me to update the software on that Era and register to the system. Once I did that, I was able to add Era 100s as surround. Thanks again!