You can only build structures within 30 tiles in a circle around your CC.
But if you either A:
Brought a communication sat when you left earth, all your robots can operate anywhere on the site.
Or B
You can build comm towers to extend their range
Only build able structures outside the CC influence is Mines and monorails. If you need to exapand, build another CC...you need at least 100 or so of every mineral and two of dozer,digger, miner, and truck.