I just got two photos taken 59 seconds apart from an AT&T camera that typically shows one bar on the daily status report. Occasionally it will show 2. Sometimes it shows none. Sometimes when I turn it off, I have to walk away from that location 40 or 50 yards to get it back online, then walk it back to its spot and turn it on, but once I get it online, it just keeps cranking out the pics.
If you have 3 bars, an antenna isn't going to do much, if anything, for you.