microcell has no ethernet connection (but ethernet works fine)

A little confusing but basically if the ethernet light on the MicroCell is not lit, and the 3G light is blinking green then the ethernet port on the MicroCell may have died, especially if there are no other issues with other devices connected to the switch. The switch could just be showing that it has a connection to the MicroCell but the MicroCell itself is not passing the connection on. Can you see if the router detects the MicroCell?

If the MicroCell has in fact died, they can't be repaired.

Just to eliminate anything the switch could be throwing in I connected the microcell directly to my router... same results as before- solid green power, solid green GPS, blinking green 3G, no light at all for ethernet.

Logging into the router to look at attached devices it does NOT show the microcell at all, by name or MAC address.

Pretty weird the network port on the microcell would just die like that... I've got ethernet stuff from the 90s that still works fine...

From what you say it sounds like nothing can be done

(shame you couldn't swap which port it looks at as I don't use the "computer" port on the thing and that'd do the job just fine if it treated it as the network port instead)

The computer port on the white MicroCell was used as a pass-through but there were so many issues with that that when Cisco designed the new MicroCell (black, DPH-154) they eliminated the Computer Port as well as the external antenna port. Like any electronic device, the MicroCell can, and will fail over time. I have an old DPH-151 that was still working just fine when I replaced it with the new DPH-154. But as they say, YMMV.

Yeah, if your router can't see the MicroCell, then there is no connection (obviously).

Try a hard reset on the MicroCell. Maybe something got scambled and you need to clear all the registers and memory and set it back to factory default. Instructions are in the Tech Guide (link in my sig).

Sounds like the Mcell is borked to me......

Yep. If the above suggestions don't work......

Same issue - spent almost 3 hrs with AT&T Wireless tech support this AM and she kept insisting that the problem was with AT&T Uverse and my Pace5268 gateway/router not having port forwarding set.  So she conferenced in the (hopeless) Uverse tech folks.  I could not get her to understand that the combination of the Ethernet light on the Mcell being dark AND the router not showing it as a connected device indicates there is a hardware error in the MCell and port forwarding works only AFTER the devices see each other.    (And, yes, of course, I checked the cable connections, swapped cables, and swapped ports on the router.)  

What are the magic words I should use when I call in again so that the issue is understood and a replacement for my DPH-151 is sent to me? 

There have been issues with the Pace 5268 gateways in the past. However, if you have an old DPH-151, I would check the ac adapter with a voltmeter first. It should read 16VDC +/-0.5. If it doesn't, then replace the ac adapter with one of the exact same rating. We've seen all kinds of odd issues when the adatpers start to fail. It's possible that the ethernet port on the MicroCell has failed as well but I would certainly check the adapter first. Just because the power light is a solid green doesn't mean that adequate power is reaching all of the board components. The MicroCell's are finicky with their power requirements so make sure you replace the adapter with the proper one. The adapters are slightly different on the DPH-151/153 and the new DPH-154. Adapter requirements are given in my Tech Guide (see link in my sig).

Port forwarding is not a requirement for the MicroCell to work properly but most of us do that anyway because it makes for a more stable environment. What is required is that the four ports listed be open on the ISP side all the way to AT&T's Mobility servers.

Thanks for the tip.  I have overvoltage:  23.9VDC.   I'll see if in my pile of old adapters Ihave one that's a nominal 16V (or 18V) with the right plug and polarity.  If not, I'll try to buy a replacement - a quick look shows me that they're available for about $7 and that gets about doubled with shipping. Any hints on where to look for less costly shipping?

Will keep you posted.  

There is an explanation of the light patterns in my Tech Guide (link in my sig). The GPS light only flashes green when it is performing a location check and becomes a solid green once the check is completed. A solid green GPS light does not mean that the location check is successful. It just means the check is done. If the 3G light is flashing red, that means the location check failed. If it is flashing green, then that could mean a failed update, a hardware failure, or something is blocking your access to the AT&T Mobility servers. The flashing internet light is obvious.

If your voltage check was that high, you could have permanently damaged the MicroCell but the only way to know for sure is to get the correct adapter, test it just to make sure, and then see if your MicroCell comes back. 16VDC +/- 0.5 with no load. The adapters are rated for 12VDC with load so one that tests at 18VDC with no load may work but the potential of damaging the MicroCell may be increased.

MicroCells can not be repaired but you may be able to get a replacement from AT&T  if you ask nicely and are polite. If they do send you one make sure you ask for the correct adapter to be sent as well because they will more than likely replace the DPH-151 with the newer DPH-154 but not send an ac adapter, and the adapters are slightly different. between the models. Again, the adapter ratings are given in my Tech Guide.