I think those are the antenna's embedded into the iPhone housing, all the iPhones from 4 and higher have that. In case you were wondering that is normal.
When I run into these issues it's usually either the cables just needing to replugged, re-alignment of antennas.
One common cause of the antenna going bad is corrosion (Liquid going to the antenna area and causing corrosion [It'll leave obvious white marks if this is the case]) or physical damage (This is unlikely in your case).
If there is corrosion on the antennas, you may be able to get away with just cleaning it with isopropyl alcohol 90%+ brushing on the antenna (Make sure you dry it up before closing the phone back up.
Check lightning connector assembly (The antenna cable that plugs from the charger port to the logic board near battery connector:
iPhone 5s Lightning Connector Replacement (I think this is the 4G one could possible be also 3G too.)
Then check logic board's antenna connector, it's the one that plugs at the top left of the board under it:
iPhone 5s Logic Board Replacement (This other one is probably Wi-Fi? / could be 3G too.)