@MissieN: Are you only experiencing network connection issues in that area, or is it persistent in other locations? Do you have access to another compatible phone that you can try your SIM in to see if the issue persists? This will help to determine if the issue lies with the phone or your current SIM/network.
@MissieN: Do you have a case or cover on the phone? If so, please try removing it temporarily to see if you notice an improvement. I understand that other phones in the household who are on the same network are not encountering the same signal issue, however as the problem is isolated to a ground floor, indoor location where signals can be weaker, it is worth exploring further troubleshooting steps.