Having trouble setting up AC1200 Smart WiFi Router (Model R6220)

> [...] We have a case where the modem [...]

   What is this "the modem"?  Maker?  Model number?  Connected to what?
(Hint: If a device has different types of Ethernet ports, then
"connected to device" is not enough detail.)

> [...] is serving about 6-7 Internet locations within about a 50 yard
> radius. [...]

   What, exactly, does "Internet location" mean to you?

> [...] Router #1 (an earlier NETGEAR model) [...]

   An actual model number would be more helpful than "earlier".  Look
for "Model" on the product label.

> [...] This then connects with router #2 [...]

   Maker?  Model?  See "Hint:", above.

> [...] This, in turn, then connects with a second trailer that lies
> about another 30 yards on. [...]

   "with a second trailer", or with some (unspecified) device inside
that trailer"?  Maker?  Model?  See "Hint:", above.

> [...] All the lights appear to be correctly illuminated [...]

   It's nice that you're happy, but actual details about LED identity,
color, activity, ... would be more helpful.

> [...] What am I missing? [...]

   A plan?  A useful description af the equipment and how it's
connected or configured?

   Based on not much, I'd expect there to be one actual router, perhaps
near your (unspecified) "the modem".  This could be connected to a
number of wireless access points (or routers which are configured as
WAPs).  Without more accurate, detailed info on the actual devices, it's
tough to provide specific suggestions on any of it.

   The router should include a DHCP server, but, if some computer
complains about "doesn't have valid IP configuration", then it would
seem that it can't communicate with that router (DHCP server) from where
it's connected.