[Singh]

Get both batteries tested and most likely replaced first. Might as well replace the fob battery.
Rear (AGM)battery is the starting battery.
You cannot hold the key to starting position to keep on cranking, it's managed by the ecu.
Also, instead of turning the key towards the right side have you turned it towards the left to start?

Once batteries are replaced as per specs then clear all codes and check starting....

 

[Singh]

With modern cars and the Touareg of course the battery condition is key to everything running properly.

Turning key to left should work all the time, well as long as one of the batteries is at optimum level.
If this isn't working for you then you have another issue.
Clear all codes and scanning again should help narrow down