This sounds like the exact same issue I have with Telus fiber. I had to put the eero behind the Actiontec t3200m router and only then would it work.

I registered just to post in this thread.  I received my Eero Pros yesterday and I am also on CCI with their 100Mb service.  My previous setup was a TP-Link Archer C9 router and a couple wireless extenders.  I had 7 SSIDs to cover my house/yard and couldn't wait to consolidate it into one network.

First off, and not really relevant to the OP, but worth mentioning, the Bluetooth pairing/set up didn't work at ALL.  I sat there for probably an hour re-trying it and letting it sit forever.  Just got the spinning loading wheel the whole time.  I got a 3-pack of Pros and tried them all; nothing.  I was so annoyed and ready to return them at this point.  Thankfully out of desperation I tried to disable Bluetooth on my phone and a button magically appeared that let me set it up by entering the serial number manually.  Why that option didn't exist with BT enabled, I have no idea.  That needs addressed because I was real close to returning my units at this point.

More to the point of the OP, once I got the gateway node set up, I received the same exact issues:
Eero Pro detected - OK
Where is your Eero Pro -- OK
Connecting to the Internet - OK
Registering your Eero Pro -- "We're having a problem" or something similar.  No error message about not being able to see the internet despite progressing past the "connecting to the internet" phase without issue.  It should have thrown an error at that point, but I'm guessing it doesn't actually try to hit the internet until it gets to the registering phase, for some odd reason.

I sat there for probably another hour trying to figure out what the heck is going on.  It wasn't until I found this thread via google search that I tried putting my original TP-Link router in between my incoming connection and the Eero gateway with wireless radios disabled.  Boom, magically worked.  This is super annoying, because I went from having a single router to having a TP-Link router, an Eero Pro, and a gigabit switch hanging off the Eero since it only has 1 output connection.  I was trying to clean up my setup but made it way messier with more cables.

I'm hoping this is resolved via Eero firmware, since it seems to be an issue on Eero's end, but seeing as this issue has persisted for 6 months, I'm not hopeful at all.  Especially since it seems to only be an issue with CCI, which I'm guessing is a tiny tiny fraction of it's users.  This is a really annoying workaround and I'm really disappointed.  If I didn't find this thread, I would have been returning my 3-pack today for sure.

Thanks for the picture and information. I will see if I can find the battery, my ONT is different.

I just purchased the eero 6 pro as an upgrade to my orbi since I have gigabit from CCI. I spent 4 hours today trying to get it to work. Called eero first and got a rude CS which pretty much told me I am screwed. I call back and the other CS person was very helpful but still didn't fix the issue but gave me some information to call CCI with. He walked me through pretty much the steps listed above. Super disappointed and believe this is something to do with the eero unit and not my ISP. Not the experience I was expecting with $600 router. Time to return them. I really wanted them to work. 

Having the same issue with the Eero pro 6 and CCII.

I just spent nearly 3 hour with consolidated communication and Eero for the same problem and could not find a fix :(

Bought it today. Spent 4 hours troubleshooting before finding this thread. Major bummer!

Yep, would be cool if Eero acknowledged this as a problem. 
 

Pretty frustrating problem to have with a $500 pack of Pros, while having to solve it by dropping a $100 router in between the incoming connection and the gateway Eero. 

Disabled WiFi on NetGear CCI router and plugged eero into it. Pathetic fix for such an expensive router

Anyone test this out on the new 6.1.0 firmware? The release notes are always super vague so who knows if this was even looked at. 

Having the same issue - any new update? Sidney eerror

I’ve been having the exact same issues. Since the whole pandemic kicked off and started working from home, became more aware of my existing network limitations with my old netgear nighthawk system. My A/V guy recommended Eero and so come and installed it all as part of other work we were having done around the house. We had a whole conversation about how quick nod easy the setup was etc etc as it was being unboxed, only to have all the issues mentioned above occur. We eventually had to put the netgear nighthawk router back in front of the Eero in order for any of it to work. First time my guy had seen it. Spend hours on the phone since with both Eero and CCI. It was mentioned by Eero that this was a known issue with CCI, that they were working on a fix etc, but there was no ETA on said fix.

Super disappointing as paying for 1gb through CCI - which we are getting to the house, but then the speed is throttled back going through the Netgear equipment and even worse once it comes through the Eero Pro 6.

What’s worse is I thought the issue might have been due to the Eero pro we were using limiting potential speeds, so I literally bought a 3 pack of the Eero Pro 6 as soon as they were available for pre-order but when they eventually arrived, no difference at all, but down a further $600 and still relying on my old netgear router to make any of it work...

Eero, if you’re reading any of this, the support just isn’t good enough. All of the support emails focus on what speeds are coming in and what speeds are possible off the Netgear equipment instead of focusing on the fix required to work directly with CCI and provide me the speeds I’m paying for. I’m so over this now which is infuriating as the Eero system is otherwise a joy to use in terms of UI, ease of adding devices etc... 

PLEASE PROVIDE A FIX ASAP....!!!

Tenarius I’ve power cycled the ONT, but wasn’t sure on the backup battery aspect. What speeds are you getting with your Eero system?

Sidney Any further updates on the situation and when we can expect a solution?

I have the same issue. Just bought a house and got 1Gb with CCI, I got the 3 pack of Eero Pro 6 but it will not setup. I had to pull out my old Linksys router and plug the fiber connection into it and then plug the eero into the Linksys. This is a crappy solution, Eero when will this be fixed? I’m debating on keeping the eero’s and hopping it gets fixed or just return them and get the Orbi system.

6.2 includes a fix for this!