Why did you even let them leave if it wasn't connecting ? Get them re-involved !!
In the meantime, when you do have a connection, what do the modem's signal stats look like ? Try getting them here http://192.168.100.1 or here http://10.0.0.1
Please post the *Downstream Power Level*, the *Upstream Power Level*, and the *SNR* (Signal to Noise Ratio) numbers.