Battery Issue? Car won't start/alarm beeping. Anti-theft issue?

I have a 2003 Hyundai Elantra with about 96,0000 miles on it. A couple of days the car wouldn’t start and I assumed the battery was dead. Also, every time I would turn the key, the alarm would do these short successive beeps. After AAA came to see about replacing the battery, he saw that the alarm would beep when the key was put in and told me the anti-theft system was preventing the car from starting, and that a new battery wouldn’t fix it. At the time, the battery in the key fob was also dead but I got that fixed and the key fob is working.

Any advice on what I should do?
Thanks

Anti-theft, yes. Try this:

  1. connect a new or a recently charged battery.

  2. Close all doors, trunk, and hood.

  3. unlock car with key, which should start alarm.

  4. turn key ON and OFF four times, and

    on the forth time, leave key in the “ON” position for two minutes.

  5. turn key to the OFF position.

Now try to start it.

Thanks for the tip. I’ll try it when I get home tonight.