HG8245Q2 Subnet Mask change

Hello,

the "Primary Address Subnet Mask" field has first 3 octets are locked to 255. If I triy setting mask to "255.255.0.0" for 192.168.*.* range I'm getting:

"Primary address subnet mask "255.255.0.0" is invalid." error.

There is a js function ( isMaskOf24BitOrMore) in resource/common/util.js?2018013020073372553184798 that stops me from setting the correct mask:

function isMaskOf24BitOrMore(mask)
{
	var i = 0, num = 0;

	if(false == isValidSubnetMask(mask))
	{
		return false;
	}

	var maskParts = mask.split('.');

	for(i = 0;i < 3;i++)
	{
		num = parseInt(maskParts[i]);

		if(num != 255)
			return false;
	}

	return true;

}

Anyone knows why is it there ?