proton38 Fellow 6a user here. :)
proton38 Anything I can do for the phone to recognised the battery
I'm not sure. I've never done a battery replacement before, but are you positive it was installed correctly? (I'm assuming it was, as you said you put the old battery back in and it works) I think that eBay batteries aren't the best quality either, so that could potentially be the problem.
proton38 or remove the update?
That wouldn't be possible. I'd recommend even just looking at a newer phone at this point, if your budget allows. I'm going to look for a replacement for my 6a around Black Friday.