Hi @dhenry4 ,
Seems as though the SIM card reader may be faulty.
The other carriers might have showed up because they operate on the same frequencies as your phone supports. However your carrier's SIM card and the fact that the phone may be 'locked' to Cityfone would have prevented you from connecting to them. As I said it was only to prove that the signal detect circuits were working and that the fault lay elsewhere i.e. the SIM card reader.
Also I would hesitate to use the solution as described in the answer above. Allowing any liquid to come into contact with electronics is just asking for trouble
A loose antenna cable may be the problem. Try writing to the SIM card i.e move Contacts to SIM card etc, that will tell you if you can read / write OK to the card.
Here is a link to the Ifixit guide repair to your phone. In Step 8 you can see the antenna cable.
Nexus 5 Motherboard Replacement