Mercedes 2005 C240 Stuck in 2nd gear

Joe Athey • 8 September 2019

I have a C240 Mercedes with 220k miles. I had the Drive in S (sport mode). I was putting along on a busy interstate and floored it to change lanes. As soon as I did this something seemed like it slipped and then it went into limp mode in 2nd gear. 
I drove it about 3 miles after that. I parked it and unhooked the battery cables and touched the positive and negative cables per the instruction of a friend. This was to get it out of limp mode and discharge any residual current that might be stored. I started it and it clunked going into reverse. I got it out on the road and it was still stuck in 2nd gear. 
I tried to drive it home about 70 miles so I drove about 50 mph in 2nd gear. Stopped to get gas and when I restarted the car it was in 1st gear. Tried unhooking the cables and let it sit for a while and could not get it to shift into 2nd, still stuck in 1st.
Took it to a friend who cleared all the codes and he told me 3rd gear is out of it and it needs a new transmission.
The car had been sitting for a week in my drive: I started it, put it into reverse (no clunking noise), backed it out and when I took off it was in 1st gear, it shifted into 2nd but still will not shift into 3rd. When I slowed down and stopped it did not go back into 1st it was again stuck in 2nd.
I have read through several articles on this page. Many state that it is probably not the transmission but electrical. Reaching out to someone who is in the know for a diagnosis.

Mercedes-Benz 220000