setting up multiple MicroCells in one house with one router

..... the beer is in the mail

I forget but is there anything else connected to the second router besides the MicroCell?

No, there is nothing else connected to the second router other than the Mcell. 

I spoke to someone at ATT technical support... They said there is no way to get the second mcell to work via my configuration and that an mcell has to be connected to the router that is connected to the modem...

Any thoughts on this?

Hmmm, the MicroCell does need some sort of wired connection to the router (and to the modem) so that could be your problem. Some have tried PowerLine Adapters (ask Avedis) but they don't seem to be reliable. A network extender may be a better choice.

Ok, I had to go back to the beginning of this thread and re-read some of it. I know it's difficult to run a cable from MicroCell 2 to the router but that may be the best, and most reliable way to connect it. You know that PowerLine Adapters don't work well and I should have said a signal booster (Wilson Signal Booster) and not a network extender, but they can be pricey.

My guess is that your MicroCell is connected to router #2, but the 3G signal is not being sent to router #1 (which is connected to the modem) because the AT&T 3G signal is incapable of being transmitted via WiFi. The only options available to you, that I can see, is either figure out how to hardwire router 2 to the base station, which you say you can't do, or try to use a Wilson Signal Booster where router #2 is and remove that router from your LAN.

Whoa there pardner!  I'm using a Amped Wireless SR10000 range extender and my Mcell is connected to it by Ethernet cable.  The range extender transmits the Mcell signal via WiFi just fine to my router.  The AT&T tech support guy is wrong.  I don't see why a second router can't do the same thing, but I only have one Mcell on my network. 

Powerline Adapters are problematic as they are subject to cross-phase, cross-breaker, ground-loop or electrical interference deficiencies in home AC wiring.  Even a GFCI can cause problems with their usage.  I tried them and my Mcell would not stay connected (flashing 3G light).  Switching to a range extender solved my need to centrally locate my Mcell in my home for complete coverage and not have to hard-wire my Mcell to my router via a long Ethernet cable.

I still think the OP's problem lies somewhere in router settings.

Have you tried swapping Mcells?  You know Mcell #1 works.  Replace it with Mcell #2 and see if it works in that location just to make sure there isn't a problem with the Mcell.

Next thing I would try is to disconnect the other Apple AC's so there are only two connected to the network, the base station AC and the AC with the second Mcell and see what happens.  This probably isn't the problem but for completeness' sake, I'd try it.

It bothers me that the second Mcell doesn't show up as a connected device.  My router shows my range extender's MAC address and its DHCP-assigned IP address.  Any device connected to my network via the range extender (like my Mcell) has its own IP address but shows the same MAC address on my router as my range extender.  I'm not sure why this is so.  One would think that if the router sees multiple identical MAC addresses, it wouldn't like it but it hasn't shown to be a problem.  Perhaps it's because the range extender has assigned separate IP addresses to any devices connected to it but then you'd think there would be a double NAT issue.  Most likely, my router has assigned the IP addresses as the range extender is in a bridge mode of some sort.  But then why doesn't it carry the unique MAC addresses through to the router?  I'm beyond my knowledge level of networking now.

I wish I was more familiar with Apple routers to say what to do but I'm not.  Perhaps you've over-complicated your network by daisy-chaining ACs together.  That should work but the Mcell is pretty finicky from what I've seen.  I'll have to think some more about this and see if I can come up with something to try.

If the second mcell does have a mac address - what would that tell us and - what would be the next steps in getting it to work?