I definitely prefer the tone in this sub. I help out when I can -- a vast majority of the issues people run into seem to be due to running a Double NAT
It's a shame eero doesn't offer guidance with adding the eero Gateway to an upstream router's DMZ. Also, it's kinda whack to sell eero secure if the eeros are running in Bridge mode; and it's hard to have happy clients when those clients' port forwarding is all over the place
Home networking seems to be a growing trend for people to jump into when they're over-their-heads because mesh networks require minimal-to-no power tools. There's still due diligence to be done and information available to access and research before you completely take down your entire home's Internet access. You are functionally choosing to do utility work
If you NEED to have another Gateway or router/modem combo upstream of your eero Gateway, this guide will be the way to go -- provided you have NO other devices connected to the Ethernet LAN of that upstream device:
Get the access information needed to log into the upstream device's GUI, and familiarize yourself with it. This is the research portion of this exercise, and will probably be the most difficult part (unless you have Sonos. Take your issues directly to them)
Set up the eero Gateway off a LAN port of your upstream Gateway. When setting the SSID, make it similar but not identical to your current SSID. Add a 1 at the end or something. If you have two networks -- one 2.4GHz and one 5GHz -- pick one. The objective is to avoid having to manually reconnect all your devices afterwards (unless you have Sonos. If you do, take your issues to their sub. eero isn't the problem)
Set the password to be the same one that you currently use
Set up the rest of your eeros, but don't run the update yet. Connect one device to the new SSID and verify Internet connectivity. Once you've done that, go ahead and run the update
Once the update completes, reconnect to your existing (old) network, preferably hardwired
Add the eero Gateway to the DMZ of your upstream Gateway, then -- and only then -- turn off all wireless radios for the current (old) network
Access the eero GUI and change the SSID to match the current (old) SSID. Then, reconnect to that new (current (old)) SSID. Verify connectivity
Power down the upstream router and the eero Gateway. Then power them back up
Reward yourself with a treat. You've done well (unless you have Sonos. You still have a ways to go yet, and it's not eero's fault)