I am unable to lock and unlock the passenger door using the method of sliding my hand behind the door handle. The drivers side is fine. I read that if the battery falls below a certain level only the drivers door will operate. A check of the battery reads 12.6v, so that is ok. I also read that turning the ignition on, flashing the main beam 5 times and pressing the brake 3 times before restarting the car will solve the issue, but that doesn't work either. This issue was noted a couple of weeks ago so took it to a dealer under warranty. On arrival the door locked and unlocked ok for once so nothing could be investigated, but by the time I drove the 8 miles back home, it was playing up again. Any ideas.

Have you tried charging the car battery to see if that solves the issue?

Just because the battery is 12.6 volts it does not mean the battery has got a lot of charge in it. 

You are correct in saying that when the battery falls below a certain state of charge the passenger side keyless functions do not work. It needs charging up above 40% for the system to start working again. 

Also carrying out the procedure you mentioned resets the battery monitoring system which may have caused more harm than good as it's only meant to be reset when the battery is replaced or fully charged at 100%. If you fully charge your battery this needs to be reset again. 

Edit: when I say more harm than I good I don't mean your car is going to break or anything like that, just the charging system and related modules may believe your battery is charged more than it actually is. 

Also if your battery light didn't flash when you tried doing this then the procedure didn't work. 

Sorry for all the info!

Car is 2019 ? Still under warranty ? Take it back and let them deal with it.