keyless start

I have a 2016 cla 250 and started having problems with my key fob not working as if the battery is dead. I would change the battery out of the key fob with a new one and it will work for a few minutes before appearing to go dead and stop working and if a change the battery out again with another...

Can anyone help me? I have a 2015 Mercedes cla 220 sport cdi and it doesn’t have the start button but I’m lead to believe they might come as standard and previous owners sometimes pinch them? Can I just buy a button off eBay and plug it in and go or is it more complex than that or is it just not...