I have a craftsman 1/2 hp door opener (model #139.53674SRT2) about 6 years old and one of the remotes stopped working. I changed the battery and when I press the button on the remote, the red light flashes on the opener so I know that it's getting a signal. I've tried the instructions listed in the manual to 'add a remote' by pressing and holding the remote button while pressing the 'SRT' button on the opener and then releasing but it still did not work. The other remote still works. Any suggestions?

Thanks