Thank you both. I know which screw is the high limit screw but when one says to "tighten" it as did all the online articles it assumes I know which way to "tighten", so initially I turned the screw counter clockwise, which caused the chain to fall off again, then remembered "righty-tighty, lefty-loosey". I turned it clockwise and it's better now. I will still have the LBS mechanics, who from this shop I trust, check and make adjustments.
Thanks to you both.

Why would this happen just after two months? Do limit screws come loose?