Active Brake Assist Functions Limited & Start Stop

Hi Guys, been scratching my head for a while with my 2016 GLA 45 AMG.

During diagnosis trying to get my start stop functionality to work as its never worked only once after a 3 hour drive it suddently began to work for 1 day. I replaced the secondary battery and still the same. Charged the secondary battery. no luck. I took it to a garage (noit mercedez) they ran some diagnostics and reported that the main battery was having issues. I never have received any fault codes on the display about a main battery and assume if it was low charge it would tell me on the display or warning/error codes. I think they reset the system and since then the Active brake assist functions limited now appears every time. I removed the 3 pin badge and there is a blank space behind with what looks like a place where a sensor should be. No wiring or anything. Looks like it has been removed before I bought the car or it was never there in the first place. I do remember the active brake assist working on the motorway as it beeped when cars were really close, but I understand there are 2 features that are similar for this functionality.

I have a feeling that he must have performed some sort of reset that has caused an issue here. Does anyones Start/Stop functionality work everyday for this particular model?

I was also told that if you disconnect the battery under the hood for 10 minutes it performs a reset which could clear up some of the issues.