Hello, I use Google WiFi and recently replaced my modem so hoping I can be helpful. What you are describing leaves me a little puzzled, so let me describe what I would have expected to happen and then ask some questions.

When you replaced your old modem, all that should have been necessary was to provision the modem with Xfinity, connect your primary Google Nest WiFi via ethernet, reboot the modem, reboot the Google Nest, and that’s it.

So, help me get some clarity so I understand your configuration and the sequence of events.

1. What was the original modem? Shouldn’t matter but trying to visualize the whole process.
2. What do you mean by “opening bridge”? Bridge mode normally refers to setting up a modem/router combo to effectively bypass the router.
3. Just to confirm, you do not have Google WiFi or Google Nest WiFi Pro, but the Google Nest WiFi, right?
4. How many Google Nest WiFi units are you using and was everything working correctly before the modem swap?
5. Which Google application are you using to manage your Google configuration? Google WiFi or Google Home? I’m assuming Google Home but want to be certain.
6. Am I understanding correctly that your PC is working when connecting via Ethernet to the Arris S33? You can see the web console and can browse the internet as expected, right? That suggests the Arris is fine.
7. Note: Xfinity doesn’t need Router information and can’t do anything with it.
8. When you switch from your PC to the Google Nest WiFi, are you using the same ethernet cable and port on the S33? I assume so. If you do, and simply reboot the modem first, then the Google Nest WiFi, the handshake will occur automatically if nothing else changed.
9. It sounds like you have done a factory reset on the (one?) Google Nest WiFi. After that did you follow the standard setup instructions (as shown here - https://support.google.com/googlenest/answer/9548301?hl=en#zippy=%2Cset-up-your-nest-wifi-router)? 
10. Could you kindly post a picture (screenshot) of what you are seeing on the Router Status Page? I’m assuming you are refering to what you see in Google Home when you go to Network > Points > Nest Wifi router > select device > select “gear icon” and see Device name, WAN IP, LAN IP, Model, and Software version, etc.

This should be straightforward, we just need to walk the sequence carefully.

You are already more helpful than Google, Xfinity and Arris combined!!

1. Original modem was also Arris Surfbaord 6121
2. You’re right. Either Google asked me to do this at one point or Xfinity suggested it
3. google nest WiFi.
4. one Router and one point which I haven’t bothered setting up since the switch. All working well before new modem
5. google home
6. Yes browsing all good
7. good to know, a Google community post with similar issue gave this advice but when I tried with Xfinity they didn’t understand the prompt  https://www.googlenestcommunity.com/t5/Nest-Wifi/Google-Nest-Wifi-Pro-Connection-Issue-The-Modem-isn-t-giving-your-Wifi/m-p/270618
8. yes same cable. I can connect my phone to the nest wifi but it doesn’t recognise the Ethernet connection. See screenshot. This is where the issue kicks in.
9. Yes several factory restarts
10. See attached. The only thing that changes is that if I refresh a few times the Ethernetlink switches to true then back to false

Again, thank you for your detailed response and questions.

Most recent attempt at help was Google asking me to also send the status screenshot (for the third time) saying that the IP and Ethernet should say “true”.

A couple of quick thoughts and then I have to run. I’ll try to check back later.

7. I see the conversation, but I’ve never encountered anything like that before.

If you plug your PC into the ethernet port on your Primary hub, does it have the have the ability to connect to the internet, and to the console? If you run a tracert to 192.168.100.1, do you see the route hit 192.168.1.1 (which should be your hub) and then traverse to 192.168.100.1? See example.

When I bring up Google Home and use that approach, I see the following, so I’m not tracking with what you are posting, or where the 192.168.84.1 IP Address is coming from.

First this - 

Then this - 

Then this (edited)

What are you seeing on this screen? Really focused on the LAN IP.

Thanks for the response. The Tracert doesn’t work on the computer.

Google gave me that 84.1 IP address to check the status of the router since I cant connect to it through my Google home app to check in the way that you illustrated.

In an update, I switched the ethernet cable on the S33 from the 2.5 Gbps to the 1 Gbps and it looks like it might be working to finally connect my Nest. The port shouldn’t make a difference… And it doesn’t explain why the other new router that I tried before taking it back to best buy (SB6190) with only one port didn’t connect. It seems so unlikely that this was the fix!

I’ll check again at the end of the day and conduct some speed tests.

Thanks for the update! Several things here that seem a little unusual. Hope it continues to work! FYI, I’ve thought of switching to the Google Nest Pros but I don’t actually “need” to, and would have to redo a lot of work. After your observations, I think I’ll be even more cautious about “If it ain’t broke don’t fix it”.