Your situation is an example where I would use different routers than the rented eero. For example, you run Ethernet from the ONT to a main router, perhaps a TP-Link Archer model. Then you run Ethernet to a second TP-Link router, which is easily set up on its webpage menus as an Access Point. Setting up an Access Point can be a pain with some routers, requiring you to determine and set several technical parameters. Not with my Archer A8; a menu just requires me to change one button from Router Mode to Access Point mode; the device sets it up perfectly, with no double NAT problems.
Or your main router could be an eero, with Ethernet running from its one extra Ethernet port to a TP-Link Archer router as just described.
(Disclosure: I am on Sonic Gigabit Fiber pre-order, and I use my TP-Link as an Access Point behind the router that AT&T fiber requires me to use. I believe this does not affect the description above; indeed, it shows that the Archer router plays nicely with the AT&T router.)