Solution for WordPress Website homepage hack again and again

Hi,

I have booked multiple domain and hosting from one service provider whenever i install WordPress any domain the homepage hacked next day.

For security reason i changed username also and keep complex password but its hacked again and again.

Whenever my site hacked then i just upload and reaplced index.php page with old one its again start working fine.

Please provide me the solution to tackle this issue or there is some problem from domain and hosting provider