Micro cell tower

Wow, and old unit.  You might try turning on Wifi calling.  Most smartphones made for ATT in the last 4 years are capable of Wifi calling. 

You need and iPhone 6 or newer, or an ATT branded android Samsung, LG or Sonim, running marshmallow or newer OS.  

As for the m-cell.  It’s possible it’s as simple as a bad AC adapter.  You can buy a replacement in several places including Amazon. 

You should plan ahead as the microcell is being phased out.  No new unit are sold, ATT expects to end Support in the near future.  We just don’t know how ‘near’.   

@OttoPylot may have more helpful information on getting the old m-cell working again.

@whatsleft - as @lizdance40 said, that is the old original model. I would check the ac adapter with a voltmeter to make sure that it hasn't failed. It should read 16VDC +/- 0.5 with no load. If not, replace it with a new one with the same rating. You might want to check the new adapter just to make sure. If you still don't get any green lights it could be that the MicroCell has given up the ghost. If that's the case, your out of luck because they can't be repaired. 

As mentioned above, sales of the MicroCell were discontinued at the end of 2017 because the MicroCell has reached its EOL. Service will continue through at least this year but after that the service itself will be discontinued or just die a slow death. No time frame has been given yet but I'd start planning now. As soon as I'm given info that I can post about the service discontinuance I'll post it.

Your available options are to switch to WiFi-C (WiFi Calling) which is what AT&T and us recommend now if your phone(s) are capable of that, purchase a cellular booster (see my Cellular Booster Guide, link is in my sig line, for info), switch carriers, or possibly try Google Voice.