Yes.. that helps loads.
Not sure which planet the Apple guy is from but he is agreeing with you to get you off the phone.
The 2nd level tech said that my 2 TB setup worked for the 2 TB since it was vintage 2014.
Is the 2TB TC the flat model.. A1409 which ceased sales in 2013? The change over to tower model was that year.. so a TC from 2014 is identical to 2018 when production ceased.
He said that Apple no longer supports connecting a Time capsule other than via the Airport WiFi, which I have NOT setup (since I do not need 2 WiFi networks). He said the newer firmware is the cause.
I can and have setup two Time Capsules in series with ethernet. Of any age and any mix. You can have old in front of new or new in front of old. I do understand the automatic setup can fail.. that is NO BIGGIE to fix.. and a level 2 support should know enough networking to help you.. !!
BTW Apple have documented how to do this for transferring the backups.. just so you know the Support guy is using his hat as a filter.
How to move Time Machine backups from one AirPort Time Capsule to another - Apple Support
I will not use exactly this method because it is going to maybe work and maybe not depending on how the old TC is configured.
- Try running an ethernet cable from the FIOS router direct to the 3 TB Time capsule to its LAN (not WAN) port and turning it on. Would it work just as an external HD (without going through Airport) and Time machine could use it direct connected?
First thing to understand.. The FIOS is your main router. A Time Capsule in bridge is a dumb switch from the ethernet ports and totally transparent.. so it makes absolutely no difference if you plug the new TC(I will call TC3TB) into the FIOS router or the TC2TB. To the TC3TB it is identical setup.
And yes, it definitely will work. Time Machine being fairly unreliable now since Sierra may still have issues.. but we can figure out most of those.
When things go wrong with the automagical setup.. which seems to happen to me more than most.. just not magical enough.. I use the manual method.
Here are a couple of
Preliminary items.
Do a full factory reset.
1. Factory reset universal
Power off the TC.. ie pull the power cord or power off at the wall.. wait 10sec.. hold in the reset button.. be gentle.. power on again still holding in reset.. and keep holding it in for another 10sec. You may need some help as it is hard to both hold in reset and apply power. It will show success by rapidly blinking the front led. Release the reset.. and wait a couple of min for the TC to reset and come back with factory settings. If the front LED doesn’t blink rapidly you missed it and simply try again. The reset is fairly fragile in these.. press it so you feel it just click and no more.. I have seen people bend the lever or even break it. I use a toothpick as tool.
N.B. None of your files on the hard disk of the TC are deleted.. this simply clears out the router settings of the TC.
2. Set IPv6 correctly.
Go to the wireless and ethernet on the computer and make sure you have IPv6 set correctly to link local only. eg..
This is IMPORTANT.. discovery of the Airport depends on the correct settings of IPv6.

3. Strange need to change to ethernet.
You may need to change over to ethernet if you have problems though.
In the airport utility now ethernet is under Other Wi-Fi Devices.. which makes no sense at all. Ethernet will only show up when it is connected and working.

So here we go.
Full manual configuration.
Plug WAN TC3TB into FIOS OR TC2TB.. makes no difference which. Plug ethernet LAN TC3TB into the computer.. this is going to require ethernet so you need a Mac with ethernet.
1. In Airport Utility
Click on File in top menu and Configure Other.

2. Type in the IP address.. always 10.0.1.1 after a reset. No password needed yet.

3. Now you can see all the parts of the setup.. you will go to each tab to do the setup.
(This example is done on Extreme and at different times.. but it makes no difference.. all firmware behaves the same contra Apple Support).

4. First is Base Station.. Here you will set the base station name and main password ..
For the least issues.. keep names short, no spaces and pure alphanumerics.. highly, strongly recommended.
Same for any passwords.

5. Next tab is Internet.. this is just left at default DHCP for now. We might need to fix the IP here for greater stability later.

6. Wireless.. on your new TC3TB there is no reason you cannot use wireless with the same Wireless Name (SSID) as the FIOS.. but up to you .. select between Create a wireless network and off.
Same rule for names.. short, no spaces and pure alphanumerics.. passwords.. same rules but can be longer.

7. Then network.. pick what you want yourself.. you are now in control.
For most people they will set DHCP and NAT or Off (Bridge Mode) For your setup it will be as screenshot.. Off (bridge mode)

8. Once you finish making changes.. use Update to configure them to the TC.. and wait for it to reboot..
If all is good it will come up on the main utility.

If this still has errors just click the TC icon and go back into the setup and fix anything there.
Any step fails tell me which one..
Take a screenshot and post it helps loads to solve where and what is going wrong.