My situation

I have a multisite setup, where I use subdirectories for different languages. I use the main homepage for English and subdirectories for other languages, as follows:

mywesbite.com : English language frontpage

mywebsite.com/jp/ : Japanese language frontpage

What I want to do

I would like to redirect people who live in Japan to the Japanese language ffrontpage without using a WordPress plugin.

What I have tried

Using the geoplugin.com , as inspired by the 3rd answer of this stack I edited the theme header.php file by adding the following at the very beginning

$a = unserialize(file_get_contents('http://www.geoplugin.net/php.gp?ip='.$_SERVER['REMOTE_ADDR']));
$countrycode= $a['geoplugin_countryCode'];
if ($countrycode=='JP'){
        header( 'Location: https://mywebsite.com/jp/' );
        exit;
    }

The problem

However, I get the following error

The page isn’t redirecting properly

An error occurred during a connection to mywebsite.com.

This problem can sometimes be caused by disabling or refusing to accept cookies.

Note: the redirecting works fine with an external website, for example

$a = unserialize(file_get_contents('http://www.geoplugin.net/php.gp?ip='.$_SERVER['REMOTE_ADDR']));
$countrycode= $a['geoplugin_countryCode'];
if ($countrycode=='JP'){
        header( 'Location: https://anotherwebsite.com' );
        exit;
    }

Is there a way to fix this error?

I am also open to other solutions not using a WP plugin.