iPhone 6s will not turn on

Hi, I have got a mate’s 6s that I am trying to fix… the phone will not power on full stop. Regardless of whether it is on charge or not! Also, a computer cannot pick it up, even a third party piece of software. I have replaced the lightning port as my first step but seems to have not worked, although I will try plugging it in to a computer to try for a response.

Is it worth trying to replace the battery or is it most likely a board issue? If so, is there anywhere in the UK that you guys reccommend for board repair or am I better off buying a second hand logic board and replacing it?

When replacing a logic board, I can put any board in right? As long as I have the associated touchid sensor?

Thanks,

Ed