iPhone 6s plus liquid damage

hi this is dinesh

i have an iPhone 6s plus. got it soaked in water by accident.

after letting it to dry for some time and tried to charge it, i couldn't get it on. then tried with a service center, they could not get it done, then went to apple service and they gave their usual answer that they won't touch phones which were opened by someone else.

then i came to some comments from friends that the logic board must be damaged or so. i could not even get a diagnostic of the problem with my phone.

can you help me get the phone fixed.