mechanical key will not open locked 2008 GL450
 

Locked the 2008 GL 450 with Key fob. Fob works to open occasionally and may need to replace key battery, which I will do. However, I need to find out for security reasons how to make the mechanical key raise the door latch.

Presently the Fob will not unlock (car battery is brand new!). Took Mechanical key out, turned 90 degrees counter-clockwise twice, nothing. Door latch (drivers side) will not raise. Anyone with an answer or solution?

MB dealer head mechanic is of no help.