Hmmm, that source is mangled by some plugin; you’d better use some “View Page Source” instead. But even if you see the true response then that will not prove that the hosting does not support POST, as somehow it does execute your PHP…
Some alternatives at https://www.thethingsnetwork.org/labs/story/thethingsnetwork-lorawan-how-to-use-google-spreadsheet-to-log-data and Save data to a cloud server.
If you want to debug InfinityFree, you could create a PHP page that only includes the following, and then use Postman to test POST with some content in int body:
<?php
// Show all information, defaults to INFO_ALL
phpinfo();
?>
When testing your actual script with Postman, make sure to set the correct content type, like application/json, to use php://input.